We knew that Phil Hughes was hurt, so file this under too early to tell, but Joe Girardi recently commented on the fear that Phil Hughes might not be ready to start by Opening Day. It's going to be around two weeks before Hughes is ready to get back on the mound again and he needs to work up to 75 pitches before he's ready to go. If that is equal to four starts than he would have to be ready to pitch no later than March 18 to be ready for April 7, the first day the Yankees will need a fifth starter.
It's obviously still too early to tell for sure one way or the other, since he could always recover faster than they expect or he could tweak his back while trying to rehab. If he does miss a start, or two, or several, who can the Yankees call on to help fill the gap? The obvious choice would be David Phelps, who will most likely starting the year in the bullpen as the team's longman and 6th starter. The Yankees give him as many spot starts as needed and in the meantime bring up a reliever like Cody Eppley or Shawn Kelley to fill his spot in the pen.
At the end of the day, this isn't really that big of a concern unless Hughes suffers a setback. He should be ready to go relatively soon and the Yankees will only have to worry about one or two starts. If he's out for any long amount of time, then it might be time to get worried.