With Gareth Bale, Marcelo and Toni Kroos all rested by Lopetegui, it was down to Marco Asensio to hit a first-half winner against his former club.
"We're not going to win 5-0 every game," the France defender said to beIN SPORTS .
"We're going to sweat and we need everyone, we're going to give everything to get the title.
"We have the ball, but it's hard to hurt the opponent. We created danger but not as much as in other games. The result is very good.
"They are a team that has many weapons and that presses well. We played as a team."
14 - Casemiro made 14 recoveries against Espanyol, his highest ever total in a La Liga game. Essential. pic.twitter.com/5IshcmhEDC— OptaJose (@OptaJose) September 22, 2018
"We knew that we were facing a great team, very hard working," Asensio added.
"It's complicated after the Champions League effort. The three points are important. In the second half our fatigue was noticed.
"In the first half we were better, we moved the ball better, in the second half we lacked patience and went from side to side.
"We go to Sevilla [on Wednesday] … that pitch is very tight. Sevilla are a great team, but we will go there to win."