jdh is exactly right. Nevin is aging, and he has never been anything close to quick, which is why he usually plays first or third. However, since Klesko took over third and Burroughs is getting comfy at 3rd, Nevin had to play OF just to get a spot back in the lineup. His bat is good enough that the Padres can stick him out there, but once they have a lead later on in the game, odds are Nevin will be replaced.
However, I would still be pretty stoked if you're a Nevin owner. His first week back has been great, including 2 homers and a .300 average. Who cares if he sits out his last AB if he already went 1 for 3 with a HR in the game? I'm more than happy with it. Expect him to keep up this pace, as it isn't ridiculous by any means, and he should have a great last couple months.
Hank Blalock is The Truth