Costa watched Atletico lose 2-1 to Chelsea from the stands at the Wanda Metropolitano Stadium in September, days after the two clubs finally agreed a deal worth £58 million to bring an end to an acrimonious transfer saga.

Atletico's transfer ban meant the 29-year-old could never have been registered in time to participate in the Champions League group stage, and he has been powerless as Diego Simeone's men slumped to third place in Group C behind Chelsea and Roma after five matches.

Nothing less than victory at Stamford Bridge, coupled with Roma failing to beat Qarabag, will see Atletico avoid dropping into the Europa League, and Luis -- who described Costa as the best player he saw at Chelsea during his own one-season spell with the Blues -- conceded the situation has been difficult.

"It is frustrating seeing him playing like an animal [in training], but he can't play," Luis said. "It's just a month until he can play with us. He'll help us a lot. I don't have any doubts about it. He knows the club, he knows the way we play.

"He has already adapted. I know it was difficult times for him and for Chelsea. Now everybody's happy. We want to see him play and happy again.

"I think he came back to Atletico because Chelsea couldn't keep him because it's his desire. He really wanted to come back to Atletico. He was happy here. He did his job really well. His most important desire was to come back."

Costa will not be back at Stamford Bridge for the match after Simeone decided to leave him in Madrid. When pressed on why he had made the decision, the Atletico manager replied: "Because I'm the manager and I make the best decision for the team."

Simeone knows Atletico's destiny in the Champions League is out of their hands heading into Tuesday's games, but he insisted it has not affected preparations.

"We need to think about our game tomorrow," Simeone said. "We're playing a fantastic team who are top of the group and they deserve to be top of the group. We don't have to look at what happens in Rome. It's not going to be a simple game, but we need to take it to where we think we can hurt [Chelsea]."

Simeone added: "You learn something from every game, whether you win, lose or draw. Tomorrow we're up against a very good team, playing at a high level.

"They played well at the Metropolitano. I hope that tomorrow we can play as we've been planning to during the build-up. We're not in the same situation we were going into the first game [against Chelsea]. We won well at home to Roma [2-0] and winning such games is always very important."

Simeone said Conte's team had earned their position atop the standings with their performances so far.

"We're in the situation we're in and we have to play our game against a team in good form, top of the group," he said. "They deserve to be top. We're only thinking about how to win the game, however, and obviously we'll try do that. There's a game of football. We need to try to win it, to do our best against a very good opponent."