Trump realized he needed this kind of guy just in the nick of time. Of course, that he’s long known Manafort, who among other connections has lived in Trump Tower for a decade, made him the perfect choice—not despite Manafort’s having dealt with so many unsavory characters, but precisely because of that.
Some have speculated that this was delayed a bit while Cory's legal problems were forefront so it would not be seen as Trump throwing Cory 'under the bus'.