What you would be doing is called "selling low." This is when a superstar player has a few bad games, his owner gets nervous and trades him for far less than he's worth.
To answer your question, McNabb and Barber will both be solid players. If you picked McNabb with a high pick I'd be a little concerned about how he's played, but you're not likely to get his real value in a trade. (If you do get a trade proposed, post it here and get opinions before you accept it.)
As for Barber, the good news is that he got carries at the goal line, so that shows that coach Fassel trusts him to carry it into the endzone. The bad news is that he wasn't successful getting it into the endzone in short yardage situations yesterday, except for the 2 point conversion.