All we know is that he left Game 5, didn't return, was taken to a local hospital for observation and was scheduled to travel with his Bruins teammates on Sunday.
There was a report from Sportsnet that the injury might have been to his spleen.
Bruins coach Claude Julien refused to speculate after Saturday's game if his top center would be available Monday night, saying only, "It's just an injury that wasn't able to let him finish the game. He may be in next game. I'm not going there."
Outside of goaltender Tuukka Rask and perhaps defenseman Zdeno Chara, there is no player on the roster who has been a bigger part of the Bruins' success than Bergeron. If he's unable to go in Game 6 and in a Game 7 if the Bruins can take care of business on home ice Monday night, it's going to be a huge blow to their chances.
Bergeron isn't your typical NHL top-line center, but that doesn't make him any less valuable. He's not going to put up huge point totals and be a threat to compete for scoring titles every year. What he does is play a stellar defensive game, play the tough minutes against other team's best players that allow his teammates to feast on more favorable matchups and wins nearly 60 percent of his faceoffs.
He's almost always the player whom Julien calls on for key defensive-zone draws.
He not only goes up against other teams' top players, he also manages to push the play in the right direction against them. Consider that in this series, the Bruins have outchanced the Blackhawks 23-17 with Bergeron on the ice at even strength. The Blackhawks have scored just two goals with him on the ice at even strength, both in Game 4.
Oh, and he's also one of the best penalty killers on a unit that has completely shut down and frustrated every power play that it has gone up against this postseason (with the possible exception of the Maple Leafs).
Put it all together, and you have one of the top contenders for the Conn Smythe Trophy.
That's a tough player to replace if he's not in. Almost impossible, really, and it would make winning a pretty big challenge. The Bruins have some depth down the middle, but none of their centers have the same sort of defensive ability and skill. And that includes David Krejci.
Bergeron isn't the only top center whose availability is in question for Game 6. Chicago's Jonathan Toews also did not finish Saturday's game, after he was leveled by Boston's Johnny Boychuk. That would balance things out a bit. But for as good as both players are, Bergeron's absence would be a bigger deal for the Bruins.
If Bergeron can't go, that means Krejci would probably have to step up into some of those minutes. Krejci has more points than any other player in the NHL this postseason, but he's not the defensive player that Bergeron is. He's also one of the players who gets slightly more favorable matchups due to Bergeron's heavy lifting.
The Bruins are facing elimination on Monday and need to win back-to-back games to claim the Stanley Cup for the second time in three years. They're going to have to do it against a Chicago team that's starting to find its offensive game.
And they might have to do it without one of the best defensive forwards in the NHL.