The fact that Tatum himself mentioned what it would be like for him to come back 50 or 60 games into the season tells me that it isn't necessarily physical; it's more mental. Brown spouting off about he has been sacrificing his whole career and that he is proving he is a #1 can't possibly help here.
Tatum also watched team chemistry after Hayward returned and I'm sure he doesn't want people to blame him. But the fact is, this team is going absolutely nowhere without Tatum. They've been a try-hard team playing other injured teams and pulling out wins. That's not going to work in the playoffs. Maybe we'll lose a few more, drop in the standings, and he'll see himself more as a help than a hindrance