I think a loan is fine regardless of whether they choose to buy him or not in the end. Either he has a good season with Atletico, in which case there will surely be takers next summer, or he does not, in which case we've at least gotten a loan fee and saved a year's worth of wages. That probably amounts to £8-10 million all in all.
If he stays in London he'll either end up on the bench or in the stands most weekends, and he's bound to lose value either way in that case, all while we're paying him to not play.