What's wrong with Rivera?

Rivera's numbers this year have been pretty great, but something just isn't right. He has gone 3-3 with a 1.58 ERA and 32 saves in 58 games. He has allowed just 37 hits in 57 innings. He has allowed just 2 homers and 11 walks. But, you probably notice that I've left out a key stat: strikeouts. Rivera has struck out just 44 batters in 57 innings, a rate of 6.9 per 9 innings. Keep in mind that Rivera has a 9.8 K/9 the previous two seasons, in '08 and '09. Mo has been just disastrous in his last 5 games, going 0-1 with an 8.31 ERA. He has 3 saves and 2 blown saves during this stretch. In 4.1 IP, Rivera has given up 7 hits and 2 walks, and has just one strikeout. Has he just gotten old, or is something else wrong with him?

Obvously, if Rivera has finally begun to show his age, that's the reason for his problems. But, I think it may be for a different reason- he just isn't as comfortable in pressure situations anymore. For his career, Rivera has a 1.89 ERA in save situations. In '10, he has a 2.38 ERA, nearly half a run higher. So, now you may be wondering how the heck Rivera has a 1.58 ERA. Simple- he has a 0.39 ERA in 22 appearances in non-save situations. He has a 2.37 ERA in those situations over the course of his career. Rivera has been way better than his career average in situations with less pressure. Should he go into a Trevor Hoffman-like role next season and be demoted to a setup man? Also, one statistical oddity is that the platoon splits against Rivera have gone a bit against his career trends. In his career, Rivera has been better against lefties than righties, with lefties hitting .206 against him compared to .215 by righties. But in '10, it has been the opposite. Rivera has dominated righties, holding them to a .151 BA and no homers. But against lefties, Rivera has "struggled", allowing a .216 BA and 2 homers. It's pretty strange. So, besides the age factor, problems for Rivera have been pressure situations, a lower strikeout rate, and some struggles against lefties. Rivera just hasn't been the near-perfect pitcher that we all recognize. Hopefully that will change in the playoffs.