The best option for defending a big guy in the post by bodying him up and denying him post position is Marcus Smart. When Walker, Smart, Brown, Tatum and Hayward are on the court together (which they will be, especially at the end of games), Smart is the inside defender and de facto Big. He can root any post player off his spot and goad him into charges. Smart also has the emotion and personality of a team leader.
Brown's defensive skills are better suited to smothering an opposing wing on the drive. Tatum's premier defensive skill is blocking shots off his man. Hayward played excellent defense on Durant in 2017, but I doubt that we'll ever see that level of defense from him again.
Smallball lies before us, like it or not.
***
To answer the thread's question, my money is on Grant Williams. The Celtics will need Bigs, and Williams is quasi-Big in the same way that Smart is: strong, intelligent, disruptive and clear on team defensive concepts. Brad Stevens played Ojeleye a lot in 2017-2018 not only because he could move his feet on defense against Antetokounmpo, but because he grokked defensive rotations. Semi might have bricked three-pointers, failed to attack close outs, and otherwise weighed the team down, but his quick feet got him into defensive position often enough to earn Stevens' trust.
Grant Williams is Semi Ojeleye with a post game, who can rebound in traffic. Grant is a Full Ojeleye, and Stevens will love him.