Grant is a tough call. He is a useful and productive back up PF. He is a versatile player overall (although fairly one dimensional offensively) so well suited to the role he has on the Celtics. I think if a team pays him like a starter and then expects him to play at starter level, they will be disappointed.
I am not sure what team is going to pay him that kind of money. They would have to have cap space and I think teams with cap space are going to have better options than Grant Williams. That leaves the MLEs. Some non-tax paying team may be willing to use that slot on him, I don't know. Again, there will probably be better or at least higher upside options. Contending teams, that may be looking to fill a bench role, are probably tax payers anyway. They won't be able to offer anything the Celtics wouldn't match.
Bottom line, nothing the Celtics can do at this point other than wait to see what he gets offered. I don't think he is going to get a crazy high offer and in the end, the Celtics will likely match.
Teams with "Projected" Cap Space per Spotrac:
SAN
HOU
IND
ORL
DET
CHA
OKC
These are the top 7, don't see any of them looking to spend big on Grant. Then there are some teams that could have cap space if they renounce current players:
LAL, UTA, SAC, MIN, POR, MEM, CLE, CHI
These teams would have to actively clear salaries by renouncing current players, essentially hard capping themselves, in order to sign Grant to the kind of money that is being thrown around ($15M-$17M). No other team is going to even have a chance to sign Grant.