I think Nady will keep getting a good amount of starts in CF even after Roberts is back. Just a better bat. Roberts can come in as a late inning pinch runner for the speedy Klesko and take CF and move Nady over to LF.
Look at Roberts double rate, about 10% of his hit are doubles. To me that means he isn't a gap hitter, which you need to be at Petco. Compare that to other fast, avergage hitting CF's (Lofton last 5 years, Preston Wilson) who have 15% and 20+% hits as doubles.
If you need a guy who can bunt his way on and beat out grounders, Roberts is your man. If you need someone who can drive in runs and get to second with his bat, Nady is it.