The latter would have put him ready and recovered sometime around August, just in time to ramp up for training camp.
"If you have an injury like that, you want to make sure it heals right, properly. Still go with 6-8 months, there's nothing I can change," the defenseman had said back on December 30.
Although, if you're an athlete, especially Seidenberg, that isn't necessarily your mindset.
A little over four and a half months following surgery, the defenseman was close to a return to game action. If the Bruins had advanced to the Eastern Conference Final, he could have made that return, in mid-May.
"I think right from the beginning I thought, I mean they do tell you six to eight months, but I know people that come back in a shorter period of time, and so I just told myself, why not?" said Seidenberg, speaking with media on May 16, for the first time since December 30. "I'll try my best and see where it takes me."
It took him farther than anyone - except for maybe Seidenberg - originally thought.
"We did everything in our power to try and get back," he said.
The training staff worked tirelessly with him. When the team was playing, and practicing, he was still around, in the gym, doing his rehab and doing as much as he could. ACL tears used to be career-ending. With the proper treatment nowadays, and the committment from an athlete, that's an issue of the past.
Seidenberg first hit the ice on April 8, when the team was wrapping up their final road trip of the regular season in Minnesota and Winnipeg.
Gradually, the skates with Strength and Conditioning Coach John Whitesides turned into skates with the team at the end of April, and eventually, he began taking light contact towards the end of the series against Montreal.
He didn't feel pressure to return, just to perform, had he been given the chance.
"There’s obviously pressure to perform because you don’t want to come back and be a liability. You want to come back and try to be at your best, but there was no pressure whatsoever," he said. "It was all about being smart about returning and being also cautious in regards of my future."
When the injury first happened, it was immediately deemed "season-ending," based on the timetable. The only talk was of the future, and of next season.
But, still, it wasn't really a surprise when Seidenberg was getting closer to a comeback.
Is there any other player who is jokingly labeled "not human" by fans, media and teammates alike? Who else can take a slap shot to the face, and not break skin? (Well, besides maybe Johnny Boychuk.) He's tough, and in excellent shape.
"I mean, it’s just putting in the work and sticking with it," said Seidenberg, asked by a reporter of his 'superhero' or 'ironman' status. "It’s a long period of time it’s taken, but if you just take it day by day, it goes by that much faster."
"It was definitely strong enough," he added. "I mean, if something would have happened it would have been another freak thing, and you never really think about it when you play…you just play."
"Conditioning-wise, it’s great. I’ve been doing lots of cardio over the last few months. Strength-wise, the same - pretty good, there’s still a little pain, but that’s just stuff you can deal with and once you play, you don’t think about it."
He never got the chance to test it in game action, and watching every game from up top in the press box made that even tougher.
"I mean, I went through this before. It’s terrible. It’s very annoying. You feel out of place. You feel you’re in somebody’s place. You don’t know where to be," said Seidenberg, who also missed the 2010 postseason due to a wrist injury.
"It’s just uncomfortable and it’s not fun watching because as I said before, you’re always the best player looking from up top on the little ice surface and seeing everything perfectly, but it’s not fun."
The Bruins could have used his experience during the playoffs.
A younger blueline that grew and found its way through the final four months of the regular season should certainly be given plenty of credit for how they filled in.
"I mean, they were tremendous, like all season, they’ve been playing great. The team hasn’t missed a beat without me, so they’ve done a great job and it was awesome to see," he said.
But really, you could never replace Seidenberg.
The strong, physical presence he brings makes him one of the toughest defenseman to play against. His best shutdown work comes in the playoffs. He was acquired after he caught GM Peter Chiarelli's attention in the 2009 postseason, when the Bruins were eliminated by Seidenberg and the Carolina Hurricanes. He was signed to his second four-year contract extension at the start of the 2013-14 season that keeps him in Black & Gold through 2017-18.
He relentlessly angles off opponents like clockwork, and his positioning is often impeccable, forming a fortress around the Bruins' net. Paired with Zdeno Chara, the duo has shut out top players time and time again.
"I mean, when you talk about Dennis [Seidenberg], he’s an important, a very important, player for our team," said Chara.
"He’s logged big minutes. He’s played all the situations. So yeah, we did feel that it was a missing piece in our lineup. But again, there’s nothing you can do about it, it’s just part of the game that players do get hurt. That’s part of what you can’t control."
"So, you give the younger guys a chance to step up, and they handled it really well, and they played really well."
The silver lining, of course, is that those young players gained experience, and now, Seidenberg can truly give his leg time to heal even further, no matter how healthy he was and how ready he was for a return.
"Right now, it’s more about keep doing rehab until it feels great, and then just keep working out," said the blueliner. "I mean, I haven’t really had a strenuous season behind me so there’s no reason why I should take some time off."
"For the most part, just getting ready for next year and hopefully feeling great."
When he does get to make his return, he won't be the same player.
"I’ll be better," said Seidenberg. "I’m confident I’ll be hopefully better than before."