I like bowen to continue starting. Udoka did great in the win against the lakers, but he needs more time to mature into his role. Defensively , Bowen is still at par with him. Bruce can defend in three different position (4,3 and 2). The best position for Udoka is the SG position and he can be rotated to the SF position for defensive purposes. He is a natural 2 and only 6'5 in height. Considering this, he can only guard players within the no. 1,2 and 3 positions, beyond that it would be a field day for the opposing player.
I think Pop is thinking of developing a third player that can be a perimeter defender. With bowen, Ginobili and Udoka rotating on the best offensive player of the other team-surely the spurs would be great defensively come playoff season.