It's ok to root to your desired team. We have our own favorites in the first place.
However, in case that happens, I don't see that as a huge success of Tacko Fall's NBA life. His name is just a part of the Boston Celtics roster but without any contribution, he will not feel literally the experience of being a champion.
And I'm not also sure if a player under a two-way contract will also own a championship ring even they are officially listed on the roster as in the case of Tacko, he mostly played in G-League.