Barcelona head coach Ernesto Valverde explained his decision to use Lionel Messi off the bench in Wednesday's El Clasico draw, saying the superstar still had some "discomfort".
Valverde said the 31-year-old was still struggling somewhat before the game and he opted not to risk him.
"The decisions you make are based on what you think is better for the team and the player," he said.
"He had discomfort. Today he was better but we thought it was best he enter in the second half.
"He could be decisive because we know the danger he creates every time he gets the ball."
La Liga leaders Barcelona make the trip to Athletic Bilbao on Sunday and Valverde is hopeful Messi may be ready to start.
He said: "I don't know. If he's okay, he'll play. I hope it's like that."
Barca came from behind at Camp Nou as Malcom's second-half goal cancelled out Lucas Vazquez's sixth-minute opener.
Asked if Madrid were the favourites ahead of the second leg on February 27, Valverde said he knew the challenge awaiting his team.
"We are forced to score in the Bernabeu, that is evident," he said.
"But there is still a lot left [before] that game. We have to be calm and we will have time to focus."