Torres is a free agent following his departure from boyhood club Atletico Madrid after Wednesday night's Europa League triumph over Marseille.
Benitez has worked with Torres before, bringing the 34-year-old to Anfield in 2007 and also managing him at Chelsea during the 2012-13 season.
"I think it will be harder for him to come, not because we don't want to," Benitez told Marca.
"I don't know what's going on in his head, but maybe he is thinking about something else. It would be attractive, but I don't think it would be easy.
"At Liverpool he was sensational. Then he went to Chelsea and, although he had a bad time when he arrived, he was essential for us in winning the Europa League."
Celtic have also been linked with Torres, as well as a number of clubs from the MLS and the Chinese Super League.