The big question is which version of Gordon the Celtics will see. If he's at or close to his career best, then he is the second best player on the team and needs to both start and end games. It makes no sense to bring your second best player off the bench. If the second unit needs scoring Brad can arrange the rotations to make sure that one of the starters is always on the floor, it can even be Gordon if that's the best way to do it.
If, however, he does not come back at or near his best, then bringing off the bench would make a lot of sense. In that situation Tatum is likely the second best on the team and putting more size in the starting lineup makes sense.