Happ actually pitched well in September. Was he a disappointment last year? Absolutely! But his record was 12-8, and 20-11 in games that he started.(I know his ERA was a shade under 5.00) His main problem was giving up the long ball. If he corrects that, he'll be fine.
When I look at this 40 man roster, I don't see any of the prospect pitchers even close to MLB ready. So, Montgomery and Happ battle for the five spot, with the loser going to the bullpen. I hope that Montgomery earns the spot. I would love to have the lefty in the rotation for many years. He displayed a nasty curve ball during his rookie season.
It's all about depth as you can NEVER have enough pitching.