It makes sense for them to loan him out as they'd get a loan fee and his wages off the books for 4-5 months. No point in paying him a fortune just to train with them.
Thats what barca did when they had their ban, Turan for instance didnt get loaned out they had him train with the squad not playing.
Atletico might think loan him out to get him fit and match ready for the second half of the season. There is no way they would loan him out for the whole year though there is no point to that.
It will either be they buy him and he trains with them until January when he can play and can spend the time he is there getting used to their tactics again and learning how to play with their current squad. Or they buy him and loan him out somewhere until the end of December so he can come back from their winter break fit and ready to go.