Donovan would be a perfect second or third option on a championship team, he's obviously capable of being the #1 option on a playoff team but I'm not sure he'll ever reach the level of being #1 on a championship team.
The same could be said for Allen Iverson, Derrick Rose, Damian Lillard, Kyrie Irving, Deron Williams and other high-powered combo-guards. Mitchell needs someone real to play next to.