He is not expected to feature under Jose Mourinho in the near future and will have a chance to pick up some first-team game time with the League One club.
Riley made two first-team appearances under Louis van Gaal last season, with a brief cameo in the Europa League and playing a half in the FA Cup.
Reports claim Sheffield United have been interested in Riley for a number of weeks but were forced to wait for him to return to fitness after his recent injury.
He’s expected to travel to Sheffield on Tuesday to finalise his move as Chris Wilder is expected to bolster his squad this month.
There’s no doubt that Riley would have been frustrated that he didn’t make it into United’s pre-season tour, and he’ll be eager of making a name for himself during his time with the Blades.
He had been training with Man United’s first-team for a number of weeks but a loan move presents him with the opportunity to help Sheffield United build on their lead in the League One and pick up some needed experience.