The 19-year-old, who is regarded as one of the best young talents in England, was not involved in the squad for Leeds's defeat to Middlesbrough on Tuesday.
"I spoke to David Moyes at Everton. He wants him to play and I couldn't guarantee him that," Warnock told the club's official website.
"He's done well, he's a smashing player, but he needs to play games and I can't guarantee that."
Barkley joined Leeds on loan at the beginning of last month, but will now return to training with the Merseyside outfit.