I am not sure if he regressed, never was that good to begin with, or just doesn't fit in Deboer's system, but quite a few players we get just seem to never fit.
I think all of the above in some measure is probably what it is. He was getting probably more opportunities in Arizona than he deserved given the talent on that team and wasn't going to get that here. He was also not a great fit because they were looking for goal scorers and instead signed a playmaker type which would be fine if the Sharks didn't have a decent amount of those already in their lineup. Maybe not necessarily fitting in with DeBoer's system is as much the problem as he didn't fit on the roster in terms of the talent and what is needed to play off of one another.