His effort had looked to have ensured the spoils would be shared before the late drama.
The result moved Atletico to the top of the table with 10 points from four games, with a number of the other sides at the top yet to play this weekend. Espanyol have two points from their four games.
"It was a really tough game, especially in the first half," Carrasco said.
"The boss made a number of changes which shows how deep our squad is and the substitutes made a difference.
"The added time was just, I thought. Espanyol wasted a lot of time, there were two VAR reviews and we were able to use that to our advantage and score."
Atletico coach Diego Simeone also thought the added time was fair due to time wasting from their opponents.
"It was a lot of minutes but I've never seen a game with so much time wasted, you'd have to look at what happened during the game," he said.
Lemar, one of those who came as Simeone made three halftime changes, thought he had levelled eight minutes after the break when he fired in Marcos Llorente's cross.
However, a lengthy VAR review deemed Luis Suarez was offside in the build-up.
In the end that lengthy pause -- one of a number in a feisty encounter -- was key for Atleti as referee Juan Martinez Munuera added on 10 minutes of stoppage time at the end of the game.
Atletico pushed on for a winner against visibly tired opponents and Lemar broke into the Espanyol box in the final minute to fire past Diego Lopez and secure all three points for Atletico.
"He's in a really good moment," Simeone said of Lemar. £He had a digestive problem with the national team and lost three or four kilos and that's why I had doubts and thought about keeping him back for Wednesday.
"Let's hope he can repeat this. He is the player we went looking for at Monaco, he is the player the team needs. He is direct, he shoots with the right and left, he works.
"He is a really important piece of the team. Last season he grew a lot but lacked goals. We hope this season he will add goals, like today."