Centre-back Ramos has been struggling with a back injury and played through the pain barrier as Real lost 2-0 at rivals Atletico Madrid in the Copa del Rey on Wednesday.
However, the Spain international will not be risked for Saturday's La Liga clash at the Santiago Bernabeu.
Head coach Ancelotti had better news regarding La Liga's top scorer Ronaldo, though, with the Italian insisting there is no truth in reports the Portugal star has a knee injury.
"Sergio Ramos will be rested against Espanyol," Ancelotti said.
"Ramos will be the only change, the rest of the team will be the same as normal."
On Ronaldo, he added: "Cristiano will play tomorrow.
"Cristiano's knee? There's absolutely nothing wrong with either of his knees."
Real head into the clash on the back of consecutive defeats to Valencia and Atleti, which followed a 22-match winning run.
Ancelotti rubbished talk of a crisis after a slow start to 2015, but wants to see his side play with more intensity.
"The most important thing is to get back to winning. We have a small advantage in the league and we want to preserve it," he added.
"I don't think we're lacking physically. Actually we need to be more intense in our play.
"Espanyol have changed from last season, they have a new style. They are well organised and played well against Valencia [in a 2-1 Copa defeat on Wednesday]."
Ancelotti also confirmed that no decision has yet been taken on whether Real will sign sought-after Stromsgodset teenager Martin Odegaard, who has been training with the European champions.
