Benitez reckons that has led to a dip in the Spanish international's form of late - with only one goal in 14 games.
"Fernando has been playing 10 games as the only striker we had," said Benitez ahead of Sunday's match with Manchester City.
"The other teams have three or four strikers. Now we have two, so we have to use both of them. The number of games and the congestion and the time you have to recuperate players, it's not easy.
"You have to have 11 players as fit as possible with fresh legs, and the right balance tactically. We have to manage.
"I can see him improving. "