Assume that Splitter only plays the minutes that Duncan sits and that Tony splits the PG minutes with Mills. That leaves 144 minutes for Diaw, Leonard, Green, Manu, and Beli. There is not plenty of room for Granger, or any free agent signee, to get extended playoff minutes. Unless you believe that he will displace one of those five players from the rotation.
Granger's a smart guy. He carries himself like a Spur and would fit perfectly in the locker room. If, however, he places a high value on the probability of playing time this season, he won't choose the Spurs.