Jonjo Shelvey will miss Newcastle's trip to West Ham on Saturday after asking to play for the club's U23s as he returns from injury.
The midfielder, who is working his way back from a long-term thigh problem, is behind Sean Longstaff, Isaac Hayden, Mohamed Diame and Ki Sung-yueng in the Magpies' pecking order after being sidelined since the beginning of January.
The 26-year-old has made only eight Premier League starts this season. His most recent appearance came in the 1-1 FA Cup third-round draw with Blackburn at the beginning of January.
Asked why Shelvey had not played since resuming training, Benitez said: "It is very easy.
"You have Sean and Hayden doing well, still you have Ki, still you have Diame - they were ahead of him and they are not in the team, so you have five midfielders now for two positions.
"We don't have the space - physically, you cannot pick more than 18 players - then someone has to stay out, and that is my problem.
"Many people say, 'Yeah but Jonjo could be [in the team]' - yes, he could be, but we are doing well."
Benitez was delighted with his response when he was told he would not be involved at West Ham on Saturday and he asked to play for the U23s at Fulham on Friday evening.
The Spaniard, who insisted the former Liverpool player still has a long-term future at St James' Park, said: "He loves to play.
"In training when you see him, he has a style of football that he loves to play, so he loves football and is keen to be there, and that is why I said it was very positive that he wanted to play with the U23s, because he wants to play.
"If he is fine, he can compete against anyone. He has no problem with that."
Newcastle travel to the London Stadium buoyed by their 2-0 win over Burnley in midweek, a victory which lifted them to 13th in the table.
With Premier League survival now in their own hands, Benitez has urged his players to stay calm as they approach the finishing line.
"We have to stay calm, we have to approach every game with the same idea that we had from day one: one game at a time, the next one is the most important, the next one is a final for us," he said.