No, I don't think so. You're probably going to give up about 12-17 ppg out of Mendenhall (now that he's the starter, you can't really count the first 3 weeks against him) and gain what in Witten (about 8ppg in my league this year)? Westy is not having a typical season, and he's always hurt. Ryan Grant isn't doing much this season either. As crazy as it sounds, Benson is your best HB option if you get rid of Mendenhall, and right now I would be starting Mendenhall over either of the other two guys.
If I were you, I'd take a look at the rest of his roster and try to package Westy/one of your TEs for Witten and a HB of his. viewtopic.php?t=456540