Sandy Alderson would like to see Matt Harvey pitch in games without pain to convince the Mets and his doctors that Tommy John surgery will not be necessary on his elbow. Alderson mentioned that an appearance or two in the Arizona Fall League might make sense for Harvey once he begins his throwing program.
D.J. Short of Hardball Talk points out that players with one year or more of major league service time aren't allowed to play in the AFL, though, and MLB's rules seem clear on that issue. Regardless of where he pitches, however, it seems the Mets will try to have him pitch somewhere within the next couple of months to test his elbow.
Before his torn ulnar collateral ligament was discovered, Harvey established himself as one of the best pitchers in baseball. In 36 starts, Harvey has a 2.39 ERA and 2.33 FIP for his career with excellent strikeout, walk, and home run rates. Doubting Harvey might be foolish, but it's worth keeping expectations in check as he attempts to rehab his elbow, a process that does not often work.