I don't see any downside to this signing. He needed a player that was starter worthy that would sign for $4.8 million. The Celtics got one.
Is he perfect? Hell no!!! But he is a serviceable big man that can give the team 24 quality big man minutes. He will play much better offensively than defensively but is an elite rebounder. He will be a turnstyle when switched onto quicker perimeter players but will provide the Celtics something they haven't had in a long, long time...some of the best offensive rebounding in the league.
The Celtics aren't overpaying a mediocre level player, they are underpaying a slightly better than mediocre level player. They won't be massively overpaying on the last year or two of a four year deal, they have the guy for probably just one year. His contract makes him a great trade asset.
And what are the other options at $4.7 million? No one associated with last year's team is someone who should be getting more than ten minutes a game...if that. Yabusele isn't an NBA player. If Theis comes back and plays like last year, he is a third string center at best. RWilliams probably will be spending more time in Portland, ME than Boston, MA.
I think the Celtics still add a vet min center and PF. Ainge isn't done yet.