Wallace has been on a pretty severe downward spiral. His raw stats were terrible last year, highlighted by his 7.7 points per game on 39.7% shooting. Of even more concern is that his points per game, rebounds per game, points per minute, total rebounding percentage, FG%, eFG%, TS%, and PER have all steadily declined over the last four years.
He declined harder and farther than expected last season, so maybe there's some hope for a small bounceback. Overall, though, he's a role player at this stage of his career. He'll still provide toughness, good defense, and leadership, but he's not a guy who can successfully be a top-three option on a team (even one as bad as the Celtics).