Honestly I think the Celtics will win this series even if Tatum doesn't play another game in it, which i'm sure he will. I even think the C's could beat either of Det/NYK without Tatum as long as no one else gets hurt, tho it would for sure be much tougher.
What worries me is whether this will linger in the Eastern and NBA finals. The C's will need to be firing on all cylinders to beta the Cavs/Thunder, if Tatum's shooting is limited the C's become much easier to guard.