I don't have high expectations for Felix at Chelsea but looking into the loan terms, I think Chelsea did a "good negotiation" considering it is not a permanent move. If he fails, they don't have obligation to buy and will not have invested in a transfer fee.. If he succeeds, Atletico may get a ROI with him and liberate a huge wage off their budget and Chelsea will get a promising player.
Nevertheless, I'm pessimistic into this transfer. We need to consider other "Atletico to Chelsea" signings into the equation like Diego Costa (who only worked for 1 season) and Saul Ñiguez loan last year. Not many good things happen to either teams.