A world-class player for free, in playing a position where we have huge doubt right now. No brainer, isn't it?
With completely outrageous prices flying around again next (world-cup) summer window, I would sign him already in January.
Sure, he would need a lot of wages, but with 0 € in transfer-sum, it will be worth it money-wise no matter what kind of sign-on he gets.
Plus here he can win trophies again, that must be a plus for him, as that is why he wants to leave in the first place.
I know people in here wants Griezmann instead, but he just stayed in Atletico out of loyalty, so how no earth would he sign for a rival when he thinks about stuff like that?