Pettersson is broken right now. There is something wrong with his wrist and it's been a problem since his injury last year.
Watching him since, he really is a shell of what he was. He doesn't shoot like he used to, and his passing and stick handling seem to be affected as well.
He has it taped every game, and it is obvious it is limiting him. Now wether or not it is a condition that will eventually heal or not is the point of concern.
Dahlin is still extremely young, especially for a defenseman playing 24 minutes a night. He shouldn't be put in the position that he is, but Adams and Grantato are trying their best to break him the way only Buffalo can with young players.
All that said, no way I'd trade Dahlin for Pettersson right now, because I am confident Dahlin will be better with less minutes and more support, whereas I am not certain Pettersson will fully recover.