When you're seeing very big frontcourts of Semi Ojeleye, Daniel Theis, and Robert Williams, it might be time to add another wing. Same goes for the line-up we saw a quarter later which had Payton Pritchard joined by Kemba Walker and Jeff Teague. Now why didn't Brad Stevens balance these out? Well that's on him. But the main point here is the Celtics have two great wings in Jayson Tatum and Jaylen Brown and that's where their depth at that position ends.
Grant Williams is a 4 in today's NBA. Semi Ojeleye as well (though honestly he should be on the bench).