He is the unquestioned Ace of this rotation even given the brevity of the period he has been this sharp. I would just not want to see us fool ourselves into thinking that makes him the Ace or the #1 of a true contending team. Buch is not as good as he has looked in the last several starts and is not as bad as he has looked at his worst.
Bring a true 1, someone that can hold down that spot for a contending team here and slot Buch in right behind him...find a 3 out of the remainder (lester, Felix, Frank, Lackey, Cook) and just like that you have a legit rotation that would be right up close to the best rotations for contending teams...just like that.
Just make sure that the guy you bring in is a 1 in the sense of being capable of leading the rotation and the pitching staff generally as well as pitching like one and you really have something.