As a McFadden owner, it might be tough to drop the guy, but unless you play in a deep league (where over 180 players are rostered at all times), then I feel it's the right move. We still haven't seen the fruition of his supposed talent, and with this injury, it looks like it's not happening this year either.
If you're in a deeper league (with 200+ players rostered), I think Fargas is the correct add, although I wouldn't want him anywhere near my starting lineup. He's already proven himself by having some level of success in 2008 and especially 2007, while Bush's success can be summed up in one Week 17 game against the Buccaneers last year. With the passing game non-existent, it'll be crucial to establish a running game, and Fargas is the guy that has done that before. Now, against those defenses, I don't really like his chances of coming through.
I'm looking at about 150 yards combined through the next three weeks for Fargas, with Bush only rushing for about 40 yards and a TD over the same time frame. If that's something you can use for your league, make the add. I'll personally be staying far away. Once McFadden comes back, the Raiders are likely to return to using him as the main back in the committee, with Bush resuming the power back role. However, I'm not confident that McFadden will be able to return to 100 percent after the surgery. If I have him in a standard 10-team or 12-team Fleaflicker league, I'm cutting him.