Atleti was heading for a first defeat of the season after Raul de Tomas gave Espanyol the lead with his side's first LaLiga goal in nine matches.

But Simeone's decision to make a triple substitution at half-time, before also turning to Joao Felix and Matheus Cunha in the second half, paid off for the reigning champion.

Yannick Carrasco fired in an equaliser in the 79th minute – Atleti's 300th LaLiga goal against Espanyol – and then set up Lemar's winner in the ninth of 10 added-on minutes.

Six substitutions were made in the second half and there was a lengthy VAR check for an earlier disallowed goal for Lemar after Luis Suarez was adjudged to have been offside in the build-up.

Espanyol scorer De Tomas complained about the amount of additional time played, but Simeone believes the hosts were made to pay for deliberately slowing down the game.

"It was a lot of minutes at the end, but I've never seen so much time wasted in a match either," Simeone said at his post-match news conference.

"You should look at what happened during the match."

Lemar dropped out of the starting line-up in place of Antoine Griezmann, who was making his second debut for Atleti, but made an impression during his time on the pitch.

Simeone explained after the game that the decision to overlook Lemar for a starting spot was down to fitness reasons following his return from international duty with France.

"Thomas is in a very good moment," Simeone said. "With the national team he had a digestive problem, he lost 3-4 kilos. 

"That is why I had doubts and thought about reserving him instead for Wednesday. I hope he can repeat those scenarios. 

"He was the player we signed from Monaco, the player that the team needs. Verticality he is good, he has a right foot, a left foot, he works had

"He grew a lot last season but lacked goals. Hopefully this season the goals will follow like today."

Atletico's late double came after Griezmann had been substituted off with an hour played after an underwhelming first game back since rejoining from Barcelona.

Griezmann had just four touches in the opposition box and managed only two shots, neither of which were on target, though nobody could match his three big chances created.

Asked what he made of Griezmann's performance, Simeone said: "I saw quite a lot in the first half that was not good from us. Espanyol were better in the first half.

"In the second we changed and looked for other alternatives to what we saw in the first half. The team had an extraordinary second half."

While Atletico has picked up three wins and a draw from its opening four games, Espanyol remains winless on its return to the top flight.

Despite the controversy surrounding the added time at the end of the game at RCDE Stadium, home boss Vicente Moreno did not feel the need to criticise referee Juan Martinez Munuera.

"I don't think about it. It may seem like more minutes or fewer than expected, but it's down to the referee. Little can change what he saw," Moreno said.

"You rarely see 10 minutes added on in a game, but I am totally respectful. I just hope it's the same when we need a goal.

"As for the other comments made [by Simeone], I am respectful of what everyone says."

Atletico, which had failed to win on its previous three league visits to Espanyol, returns to action on Thursday (AEST) with a home match against Porto in the Champions League.