NFL Players play football, the coach has to decide which players put the team in the best place to win. You could sit a player for a sore hamstring for 2 weeks and then he gets leg wipped to a broken leg the first snap he is in the game. Jones did not look good in his comeback game, I doubt McCoy would have looked better in any way.
DJ is no Eli... almost no-one is an Eli with his ability to play every week. The team needs a competent backup, not a long shot rookie nor a never was veteran who cannot take the offense and make it work.