I think the Pirates used 23 players in 18 innings earlier in the year just before they played the brewers, and if i remember correctly, the brewers allowed the pirates bullpen to rest up nicely...
Thats exactly why I'd stick with Sheets. Give him the chance to finish what he started. He didn't quite get it done but ignoring the hindsight, with the way he was pitching, I don't know how you could take him out.
IMO, with a 4 run lead and a shutout going, you've gotta keep him in unless his pitchcount is 120+ EDIT: until he gives up the run or gets into huge trouble, that is. EDIT AGAIN: especially considering how dominantly he was pitching.