I don’t think his body will be crumbling anytime soon. It’s the World Series, people. Mariano Rivera has always pitched more than just the ninth inning in World Series play, and no matter what people say, has shown no signs of wear and tear.
Via Anthony Mccarron:
…he is convinced he can go two innings at a time for the remainder of the series, if it's necessary. "Definitely," Rivera said.
Also, if you were Joe Girardi, would you trust Phil Hughes right now? How can you trust a guy who hasn’t been the same dominant force everyone witnessed in the middle of the season?
If not Rivera or the starting pitcher, Joba Chamberlain or David Robertson should be pitching the eighth inning of a close game.