Imagine you have three starting pitchers with the stats below. The problem is that you have room for only two in your rotation. Which do you send there? (Italicized stats are projected.)
Pitchers 1 and 2 have consistently contributed more to their teams than pitcher 3 has. And, looking into the future, pitcher 1 has the strongest projection as a starter. Pitcher 2 has the weakest, but a decent track record over the past two years. Pitcher 3 has the middle projection, but has had trouble in the past.
Based on these numbers, the best idea is to definitely keep pitcher 1 in the rotation, where he will pitch the most innings. Pitcher 2 probably gets the nod over pitcher 3 because of his past two years, although that's a more difficult call based on the pitchers' projections.
So, who do the Phillies send to the bullpen? Pitcher 1 of course. Pitcher 1 is Jon Lieber, pitcher 2 is Jamie Moyer, and pitcher 3 is Adam Eaton, the oft-injured $24M man.
With Eaton's injury history (according to Steven Goldman at Baseball Prospectus, Eaton "can be injured by such disparate forces as ennui, schadenfreude, and onion dip"), it probably won't be long before Lieber moves back to the rotation. At that point, the Phils will have their strongest group of starting pitchers. Before then, they won't.