26 goals and 72 points in the 114 games prior to injury
Works out to about 19 goals and 52 points per 82 games
About a third of that production was on the PP, so take that away and it works out to about 13 goals and 35 points with no PP time
Since coming back he has 23 goals and 86 points in 256 games
Works out to about 7 goals and 28 points per 82 games
3 PP goals and 15 PP assists, without those numbers he's averaging 6 goals and 22 points per 82
If I'm going to speculate here, the injury obviously had something to do with it, at least right away. His first season back again was 2019-20, he was on pace for 7 goals and 29 points in a full season, but it was cut short due to the league cancelling for covid. Obviously a stark decline from the 12 goals and 22 points he put up in 32 games before the injury the year prior.
After that, Evason took over at the end of that 2019-20 season. Dumba was never put back into the spot that Boudreau had him before the injury, but we already saw it wasn't 100% the decline in PP production that caused the overall decline in production, so it's probable that Evason had him focusing more on his defensive game, we know Evason likes to play a stingy game. And it culminated this year in 14 points in 79 games, and we know there was a heavy emphasis on using him defensively, and a heavy emphasis on his defensive errors.
I don't know if he'll ever be a consistent 50-60 point guy again, I wouldn't put money on it, but if we assume the injury isn't still holding him back, I think the only other logical conclusion is that Evason/the system is holding him back. So will we see him get back into the 30-40 range if he goes to a more offensive system/coach? I wouldn't be surprised.