Dan Szymborski, whose ZiPS projections are the best in the business, has a take on the predictability of A-Rod's decline, featuring a projection of his performance over the next five years. Here's a sample (warning, you may want to have a barf bag handy):
Projections (BA/OBP/SLG, HR)
2013: .256/.339/.425, 16
2014: .251/.328/.405, 13
Years 3, 4 and 5 are even worse.