There is going to be plenty of rethinking Grant. I think the core issue is that Grant didn't want to be the 4th big on the Celtics. He wanted to go to DAL. The Celtics could have still matched, Grant would have gotten over it, but I have no remorse over it. $53M for a 4th big who didn't even want to be here? And who wasn't all that good anyway?
Of course now with trading Rob Williams, the big depth changes and Grant would be the 3rd big and would have a more significant role. Not a starter like he is expected to be in DAL, but still a regular rotation role. The problem for me is that I don't think he is any better than the tier of players that includes Pritchard, Hauser, Brissett, etc. Grant is an undersized PF with limited athleticism but a decent 3 point shot, so long as he is open and in the corner. Brissett is an inch taller, little more wingspan, more athletic and versatile, but less reliable from 3.
Bottom line for me, I am concerned that we need another decent PF/C to replace Rob Williams but I am still not convinced that Grant is any better to be that player than Brissett. Neither is really what is needed.