The United youngsters initially joined the Foxes until December but will go back for another month, although Keane's return may have to wait as he continues to recover from a hamstring injury.
The 19-year-old picked up the injury on his debut for Leicester against Bolton and is still with United having treatment.
Meanwhile, Lingard represented United's under-21 team on Monday night against Southampton and will return to the King Power Stadium having already made four appearances for Leicester.
"It was a tremendous education for Michael," explained Under-21 coach Warren Joyce to MUTV. "He went into his first game at Leicester playing against Kevin Davies and realised some of the things we've been saying to him over a 12-month period. You can't buy that sort of experience to play against people like that."
The duo will stay with Leicester until January 5.