Adding a solid #2/#3-type is critical. I think Roark could be a nice bounce-back guy. He's gone from really good to pretty bad back to really good again in the past, and he's still relatively young and there are no apparent health issues. He surely is highly motivated to get things back in the right direction so he can eventually cash in on a nice free agent contract before he's done with the game. I just believe in Tanner as a competitor.
Now, I'm not sure you can bank on Roark rebounding as your #5 AND just go with the better of Ross/Fedde as your #5. I'm more inclined to pick up a steady #4 to round out the back 40% of the rotation with Roark, and have Ross/Fedde as depth. Ross might be a good long guy to have to limit his innings, and then you can stretch him out if need be when a starter is on the DL. I hope Fedde can emerge, but I wouldn't pencil him in for anything more than AAA depth.