The Frenchman brilliantly set up Casemiro with a beautiful backheel as Madrid went two points clear of rivals Barcelona who drew 2-2 with Celta Vigo on Saturday.
"It's a very important win," said Benzema. "I can't say we don't look at Barcelona, of course we do. But we're focused on our own league. There are not many games left and every game now is a final."
Zinedine Zidane said Eden Hazard took a knock to his ankle.
"He took a big hit but it's just a knock I think," said Zidane.
Espanyol were far from outplayed and might have been ahead if they had capitalised on a series of forays down Madrid's left in the first half.
With Marcelo repeatedly caught upfield, Wu Lei was able to sprint clear and his early pull-back was met by Sergi Darder at the back post, only for his shot to be blocked by Dani Carvajal.
Madrid were sloppy but created chances. Sergio Ramos headed over Casemiro's smashed effort across goal.
Ramos flicked on Marcelo's crossfield pass and Benzema, moving away from goal, backheeled off the bounce through the legs of Bernardo Espinosa for the arriving Casemiro to finish.
Madrid now turn their focus on a tricky home fixture against Getafe on Thursday as they hope that Atletico Madrid inflict more pain on Barcelona on Tuesday.
Earlier, Villarreal continued their surge towards Champions League qualification with a 2-0 victory over Valencia, inspired by Santi Cazorla.
The 35-year-old found himself under a 60-yard punt from his goalkeeper before cushioning the ball out of the air perfectly for Gerard Moreno to volley in.
Paco Alcacer also scored against his former club as Villarreal moved just three points behind fourth-placed Sevilla.
La Liga Results
- Espanyol 0-1 Real Madrid
- Granada 1-2 Eibar
- Villareal 2-0 Valencia CF