The former Liverpool boss is in charge of the Napoli side that faces the Spanish team in a Champions League qualifier on Wednesday night.
And he says the side they come up against will only just cope without United's £34million summer signing, who tasted defeat on his competitive debut against Swansea on Saturday.
"He is a great player," Benitez said. "And his absence will surely be felt, but the strength of the team remains in its unity."
