Well he started 154 games and 1128 innings, which is only about 7.3 per game. This year he has started 116 games and played 900 innings, which is about 7.7 per game. So he's missing about an inning and a half per game.
I use "rest starters" when down or up by 4 in the 7th or later. I also use defensive subs when up by 2 in the 8th or later. He's set to come out for rest and also for a defensive sub.