He's got nasty stuff but would he be a better replacement for Rivera? I honestly think at this point we need to test the waters with him as a starter because our other prospects stink so far this year.
In the ideal situation I would definitely keep him as the setup man until Mo retires. However, the situation NYY is in right now isn't ideal by any means. I'm not saying that he would be a bad starter - he might be a very good starter. However, I think he has much more long-term potential as a closer.
While I can't imagine Joba as a starter, you are right in the sense that we do need him. Unfortunately, the Yankees only have 2 quality starters, Wang and Pettite. And taking Joba out of the bullpen would also hurt our bullpen since frankly, it sucks. We can't trust Farnsworth or Hawkins to get the ball to Mo.