Clubs are allowed to choose from 25 players for Premier League games.
But Benitez said Roberto Di Matteo had used just 20 - despite gruelling domestic and European campaigns.
The Spaniard, who has only managed two 0-0 draws since taking over, said: "My idea is to use the squad.
"We had too many players playing almost every game, 11 or 12 playing every game.
"Others hadn't had too many minutes. You cannot play two or three games in eight days and be fully fit. The players were getting tired."