Atletico threatening to play Aguero can, to me, only mean one of two things:
1. They are trying to rush City into making a big offer; or
2. Aguero is pretty much a Juve player and Juve, as part of the deal, have agreed to let Atletico use Aguero for these qualifying games since Juve don't care if he's cup-tied.
No.1, clearly, is more likely.