Williams impressed in the Summer league, but he's the kind of player that may not be so impressive against all the other good bigs in the league. Tacko, on the other hand, is physically dominant--so tall he can reach over everybody else near the basket--and strong enough to keep his position. He stands a better chance against the other bigs in the league--at least inside. Outside is probably more of a challenge for him. Teams will try to keep him away from the basket. It remains for Stevens to utilize these guys in ways which play to their strengths.
Tacko could well make the roster, if only because he seems to be the most popular Celtic right now. And Wyc knows he will sell tickets. I read that he isn't playing for Senegal because he wants to focus on playing for the Celtics.