Diego has previously enjoyed a successful loan with Atletico.
"It's true that I still want to return to Atletico, I spent a memorable year there and even won a trophy," he said.
"But it's also true that I haven't signed anything with Atletico, or any other team.
"It's difficult to evaluate because it's all in their hands. Atletico have to decide what they want to do, how much they want to pay and then see if they can strike a deal. It's up to them."