But with the Liga club under a transfer ban until January, Costa's immediate future appears to lie at Stamford Bridge, with Chelsea including the Brazil-born Spain global in their Premier League squad on Friday.
Chelsea have named a Champions League squad without Diego Costa despite naming him in their Premier League squad last week.
The 26-year-old Belgium global insists he is happy with life at Stamford Bridge but admits playing in Spain's top flight is an attractive proposition.
There were rumours of loan moves to Las Palmas and Everton, but ultimately, no deal was struck and Costa remains a Chelsea player.
Costa said he wanted to rejoin Atletico Madrid, and that Chelsea were treating like "a criminal" by demanding an "impossible fee" for him. Costa's registration offers the possibility of a shock lifeline but, more realistically, it is the latest move by Chelsea in a possible legal battle.
"For me personally I haven't won the Champions League so maybe that one would be a little bit more special, but I would prefer us to win everything". I hope for him he can find a solution and I want to see him on the pitch soon'. "It was a pleasure to play with him".
Mourinho said: "He will not be ready for the group phase of the Champions League, I don't think there is any chance of that. I was happy. But when the manager does not want you, you have to go". We are looking forward to playing together.