Bergeron too me is the top two-way center in the league but the Bruins managed to go 9-6-1 without him and a year ago they were like 11-5-1 without him.
The bruins are a a team that rely's heavily on a defense first system, a top PP and a dynamic first line and two top goalies. You take Bergy out put in Krejci and you still have a dynamic first line,by having two world class wingers on it, you still have one of the best PP, along with a defensive oriented system and two top goalies.Which is good enough in todays NHL to get by and you still don't have any secondary scoring.
If the Bruins actually had two scoring lines they'd be a top the NHL standings or near the top and a loss of Bergy then would be a lot more drastic because they would plummet in the standings, but as their currently built their going too finish anywhere from 6th to tenth in the east with or without Bergy.
No doubt their a better team with Bergy but with him in the line-up or not having him in the line-up, their still a one line team that rely's heavily on the top line, the PP, and a defensively oriented system with two top notch goalies.