No. 2 Boston University men’s hockey coach David Quinn had no choice but to play the cards that he was dealt.

Looking to bounce back from a miserable 10-21-4 showing in 2013-14, Quinn and the Terriers (27-5-5, 14-5-3 Hockey East) entered the 2014-15 season with a bevy of talented newcomers and a strong veteran crew, but the makeup of BU’s defensive corps looked flimsy at best on first glance.

Junior captain Matt Grzelcyk and sophomore defenseman Doyle Somerby were set to man the blue line once again, but the Terriers were forced to shore up the remaining four vacancies on defense with a group of talented, albeit inexperienced, freshmen.

Like it or not, the Terriers’ season would live or die based on whether this motley crew would be able to shoulder the responsibility of being counted on as the last line of defense.

So far, so good.

Freshman defensemen John MacLeod, Brandon Hickey, Brandon Fortunato and Brien Diffley have risen to the occasion during the 2014-15 season, helping BU boast the 12th-ranked defense in the nation, allowing 2.26 goals per game. Now they find themselves in a position to capture the program’s sixth national championship after earning a berth in the Frozen Four.

This quick turnaround hasn’t been much of a shock for Grzelcyk.

“When you have four freshman D, you can be a little skeptical to start the season, but no, I haven’t been surprised,” Grzelcyk said. “We were just hoping that they were going to be really mature, and they have definitely been that and all more. They’ve been a huge key to our success.”

And the ceiling isn’t closing in any time soon.

“Hopefully they get better every year,” Quinn said. “That’s our job, our responsibility. All of them have elite qualities about them. You’re talking about four guys that, to me, could be elite defensemen at our level.”

Usually skating alongside Grzelcyk on the Terriers’ first D-pair, freshman John MacLeod’s simple, stalwart style of play serves as the proverbial yin to Grzelcyk’s speedy, dynamic yang. While Grzelcyk often brings the puck up to the slot on a whim, the defensive-minded MacLeod is more than willing to sit back on the play, camp out on the blue line and dole a heavy check out on open ice.

For the Dracut native, it’s not about making the highlight-reel play. Rather, it’s about preventing the opposing player from orchestrating his.

“That’s the way I’ve always been,” MacLeod said. “I’ve never really been one to jump up in the offense that much, but more of just playing down low in the corners, being as physical as I can and just moving the puck up to the forwards, making the easy pass.”

MacLeod’s stingy, stay-at-home mentality has served as a key component in the success that he has achieved so far in his career.

After two seasons with the United States National Team Development Program, MacLeod was drafted in the second round (57th overall) of the 2014 NHL Draft by the Tampa Bay Lightning — the highest that a BU defenseman has been drafted since Adam Clendening was snatched up by the Chicago Blackhawks in the second round (36th overall) of the 2011 NHL Draft.

Squaring up against skaters that were three to five years older than him proved challenging at first, but MacLeod — possessing a 6-foot-1, 203-pound frame — is certainly no slouch either.

Many opposing skaters have been forced to bear the brunt of MacLeod’s pulverizing checks, with the freshman pacing the team in penalty minutes (54) on the year.

It might be easy to characterize MacLeod as just a bruiser on the blue line, but Quinn noted that the freshman brings much more to the team than just a physical presence.

“He moves the puck well. He shoots it as hard as anyone we have,” Quinn said. “He’s got deception to his game, and he’s a well-rounded player, but I think when a guy is as physical as John is, people don’t look deeper into the player, and he’s got a good skill set.”

MacLeod is currently second amongst Terrier defensemen in plus-minus (+19), third in blocked shots (49) and tied for fourth in scoring (nine points) — a testament to his all-around skillset.

Expectations have been high for the hard-nosed defenseman since he first set foot on Commonwealth Avenue, but any added pressure that comes with his draft position or BU’s pursuit of a national title has done little to sway MacLeod from his pragmatic, tenacious roots.

“If it ain’t broke, don’t fix it,” seems to be the mantra for MacLeod. So far, it appears to be paying dividends.

According to Brandon Hickey’s defensive partner, Fortunato, and freshman center Jack Eichel, Hickey possesses the best one-timer on the team. That’s saying an awful lot, considering Hickey is a defenseman on a team that ranks second in the nation in offense.

Hickey’s five goals are the second-most among defensemen on the team, and he’s second for defensemen behind Grzelcyk in shots on goal, with 111 on the season. Hickey said he likes to get shots off from the point, and has been working on his offensive production all season, jumpstarted by Fortunato’s passing ability.

But Hickey, a third-round draft pick of the Calgary Flames, doesn’t think his offense is the best part of his game. Certainly, his shot has grown more powerful and more accurate over his past two years in the Alberta Junior Hockey League and at BU — his juniors coach helped him refine that.

At the end of the day, his best abilities, he feels, lie in his roots as a defenseman.

“I think my defending is my biggest contribution,” Hickey said. “I like playing d-zones. Not very many people do. I take pride in my d-zone play.”

That’s a believable statement. Hickey leads the team in blocked shots, with 87 on the year — a significant margin over Sombery’s 50 blocked shots.

His talents don’t stop there. Grzelcyk said he views Hickey as one of the best skaters on the whole team, as a physical player and as a complete, all-around athlete.

“We knew coming in that he was going to be really good,” Grzelcyk said. “He’s definitely lived up to the hype.”

Quinn, a former elite defenseman himself, perhaps put it best.

“He pretty much can do a little bit of everything,” Quinn said.

Brandon Fortunato — unbeknownst to him until recently, he said — has yet to record any penalties on the year. In fact, he’s the only full-time player to finish the regular season with a clean record since legendary BU coach Jack Parker did so twice in 1966 and 1968. That’s certainly not terrible company.

Quinn has joked on more than one occasion that it’s pretty easy not to get any penalties when you’re sporting a 5-foot-10, 150-pound frame. Indeed, Fortunato isn’t the most intimidating presence on the ice, and he admits so himself. He’s the youngest defenseman on the team — five days younger than MacLeod — and the smallest by about 25 pounds.

But for Fortunato, it’s not about being the strongest guy out there. It’s his other skills — his stick-handling, his speed, his puck movement — that make him a significant contributor, particularly on offense. Fortunato ranks second among BU defensemen in points, with one goal and 17 assists to his name, including three postseason points so far.

“Fortunato is small, crafty, poised, generates an awful lot of offense with his breakout passes and ability to lose the first forechecker,” Quinn said.

Fortunato himself put it simply.

“I just try to use my brain,” he said.

The Terriers’ captain sees a lot of himself in Fortunato, too.

“Brandon Fortunato is someone that I try to take under my wing a little bit, because he has some similarities that I used to have coming in,” Grzelcyk said. “He has really good vision with the puck. He has really good hockey IQ. He’s really aware of what’s going on.”

The Terriers are 11-1-2 when Fortunato records an assist. Fortunato isn’t one to go too aggressively toward the net — he has just 29 shots on goal all season, one of the lowest totals on the team. But he’s always willing to chip it off to his power-shooting defensive partner.

Fortunato and Hickey have grown close as the season has progressed. They each understand how the other plays the game and how the other thinks about the game.

“We think about the game the same kind of way. I always know where he’s gonna be, he always knows where I’m gonna be,” Hickey said. “We know each other’s tendencies because we’re really good friends on and off the ice, so that helps too.”

They have several superstitions and keep each other loose before games to prevent themselves from getting too nervous. They also like to joke around — Fortunato likes to say he’s a model for Calvin Klein. (Three of his teammates have said he’s one of the worst dressed people on the team.)

And those superstitions? Well, they’re keeping quiet on those.

“They’re secrets,” Fortunato said with a laugh. “We don’t wanna let those out.”

When describing freshman defenseman Brien Diffley’s skills, Grzelcyk didn’t shy away from praise.

“Diffley is probably one of [our] best skaters,” Grzelcyk said. “Just his edge work, and he looks so fluid out there, it looks like he’s not even trying sometimes.”

Pretty good praise for a skater that was viewed as a fringe Division I prospect just a few years ago.

The 19-year-old blueliner has been yet another efficient cog in the Terriers’ defensive corps, compiling nine points and a plus-eight rating while suiting up in 38 of 39 games this season with the scarlet and white.

While the Burlington product’s transition up to the collegiate ranks has been mostly seamless, Diffley’s journey to get to where he is today strays off the path taken by MacLeod, Hickey and Fortunato.

Unlike his other three blueliner classmates, who entered college fresh off stints at impressive programs such as the USNTDP and AJHL, Diffley instead showcased his skills on the local circuit, helping Burlington High School capture the MIAA Division I State Championship in 2012, before switching to Buckingham Browne & Nichols in Cambridge.

Despite an impressive tenure with the Knights, Diffley still appeared to fly under the radar of many recruiters, but eventually caught the eye of associate head coach Steve Greeley. Enamored with Diffley’s proficiency in both maneuvering past forecheckers and kick-starting transition plays, the Terriers eventually secured a commitment from Diffley in January 2014.

It could be easy for Diffley to use his apparent also-ran brand as a source of motivation, but harping on the past is not something that the defenseman devotes much time to.

“I don’t really think about it like that,” Diffley said. “I definitely wanted to prove to some people that I was a good player and whatnot, but I’ve just got to focus on the game and focus on getting better every day and your play will show out on the ice.”

Despite playing for most of his life as an outlier from the traditional college hockey track, Diffley has proven so far this season that he certainly belongs alongside his Terrier teammates.

“He skates so well, has good puck skills and we’re not surprised that he’s had the success at our level,” Quinn said. “He just keeps getting better and better … He’s meant an awful lot to our team.

“There’s been plenty of nights where he’s probably been one of our top defensemen, if not our best defenseman.”

When you’re not alone in your challenges, Quinn believes, that makes things that much easier.

“Maybe having four of them took a little pressure off each other,” Quinn said. “There was a comfort level for all of them, saying, ‘Alright, there’s three more of us.’

“I think there’s a little bit of comfort in knowing that you’ve got three other guys in the same situation that you’ve been.”

The quartet has already played mightily through a number of high-pressure situations — a Beanpot championship, a regular-season championship, a Hockey East Tournament title and a berth in the Frozen Four.

Now, with the Terriers set to face off against the University of North Dakota on Thursday with a spot in the national title game on the line, the four youngsters will be on their biggest stage yet.

But don’t count on added nerves under the bright lights of TD Garden.

“I don’t think there’s any more pressure on the defense going forward,” MacLeod said. “I think we just need to do what we’ve been doing all year.”