It's often been said that a long, tortuous NHL season falls into three categories. There's the first couple of months, which are a glorified extension of the the training camp and exhibition schedule; followed by the dog-days of December and January when the lousy teams settle out.
Then there's the last 25 games, where teams are gearing up for the Stanley Cup playoffs and bring energy and desperation night after night. It's those last 25 games that give you a pretty clear indication of where your team and your players are really at.
So I suppose it is possible that Pettersson is battling through some kind of injury. I would guess we'll find out after the season wraps up tomorrow night. But more likely, having never played an NHL schedule before and facing top checkers every night, he's just worn down.
So my point is that he's going to have come back next year a lot stronger, with more stamina, just to replicate what he did in the early part of the schedule this season.