Bobby Valentine announced earlier today that Carlos Silva will miss his start tomorrow due to shoulder inflammation, and has been taken out of consideration for the 5th starter role. According to Peter Abraham, Valentine also said that Silva's contractual status is being discussed -- so don't be surprised if he ends up cut relatively soon.
With Bard the assumed 4th starter behind Lester, Beckett, and Buchholz, there is still a glut of pitchers in contention for the 5th spot.
Andrew Miller, Michael Bowden, Felix Doubront, Vicente Padilla, and Alfredo Aceves are probably among the front runners, with Clayton Mortensen (acquired for Marco Scutaro) and Justin Germano (starting in place of Silva tomorrow) as more long shots.
Were it up to me, I'd like it to come down to either Bowden or Doubront, if either of them could step up and show something. Padilla and Aceves could easily shift between bullpen and starting if whoever claims the 5th spot can't hold it. I'm about done with Andrew Miller. Unless we're playing the Pirates or Astros, he can't get anyone out.