Aces and #2's are always hard to find, especially through trades. but they do happen.
Greinke was traded before the 2011 season.
Gio Gonzalez was traded before 2012 season.
Many others worked out okay or did not work out very well; AJ Burnett, Shields, Dickey, Jimenez, and I'm sure there are others I am forgetting.
I realize that sometimes the cost of getting a 1 or 2 is way too high, and all that is left is to try and upgrade your 4/5 slot, but in my opinion, every effort should be made to avoid these types of situations.