From what I've seen, I don't think that there's much that Tiago Splitter could have done on Sunday to have curbed the abuse that the Spurs bigs took in the paint. I'm intrigued by Tiago's size and I think he can play. But I also think that he plays best when things are loosy-goosy schematically. He scores best in scramble situations and he's most effective defensively (to me) when he doesn't have to worry about rotations and such; when the game slows down and requires that players stay true to a scheme, he's much more of a thinker than a player and, in those situations, I think he's relatively passive and foul-prone.
With that said, I don't think it would be advisable to run Splitter out there on Randolph at any point in time, because I think he lacks both the mobility and credibility (with officials) to be effective in that situation. So, if you pair Splitter with Duncan, you're asking Tim to play extended minutes defensively against Randolph. While I have no doubt that Tim could handle that task and could probably stay out of foul trouble, I think you'd lose a lot from him on the offensive end and I think you still run the risk that Randolph will take away your one real hope in the paint with a couple of strong moves and a favorable whistle (particularly if, for instance, you get an official like Steve Javie in Game 2, who doesn't mind pissing off a home crowd). At that point, you're trying to deal with both Gasol and Randolph with the offensively-challenged and not exactly "credibile" quartet of McDyess, Blair, Splitter, and Bonner.
Still, if the same continues in Game 2 -- if the Spurs can't hit shots from anywhere and the Grizzlies are effectively pounding the ball into the paint, I'd hope that Pop would use Splitter in situations where he could check Gasol for periods of time -- mostly when Duncan sits, and particularly if Gasol is in the game and Randolph is not (there was a 3 minute stretch of the 1st quarter and a 3 minute stretch of the 3rd quarter when this was true; there was also a :50 stretch of the 2nd when Gasol was paired with Haddadi).