Florian Lejeune will return to Italy for surgery on his knee.
The Newcastle United defender ruptured the anterior cruciate ligament in his left knee against Crystal Palace last weekend.
Lejeune had made a first-team comeback from the same injury on his right knee in January.
Professor Pier Paolo Mariani operated on Lejeune in Rome last August.
And the 27-year-old will see the same surgeon for surgery on his left knee.
Lejeune could again be back early next season, according to Benitez.
“Hopefully, it will be between four and five months if everything is fine,” said United’s manager.
“But you have to wait and see what the surgeon is saying. It’s bad luck.”
Lejeune went down in agony after challenging Andros Townsend in last Saturday’s 1-0 defeat at St James’s Park.
Asked about Lejeune’s reaction to the injury, Benitez said: “It’s OK. The first couple of days he was a little bit down, but he knows what he has to do.
Lejuene played well in a back five with Jamaal Lascelles and Fabian Schar after returning from injury.
Asked if he would consider reverting to a back four, Benitez said: “Every game, we’re thinking ‘are we OK if we change this?’.
“We analyse that. Sometimes we decide the day before, because we analyse the options that we have – or we analyse the other team.
“You analyse as much as you can and you work with your players with an idea.
“Can we play with four at the back? Yes, because we have been doing that the whole season.
“Or we can carry on with five, changing the players.”