Boss Nigel Adkins has given permission for the defender to join Fleetwood Town on loan for the remainder of the season.
The Blades do have a 24-hour recall in place in case of injury problems at Bramall Lane between now and the summer.
"Bob hasn't played much football this season, but he is now fully fit and needs games," Adkins said.
"We still have a big squad and, if necessary, we can have him back at 24 hours notice."
Harris, 24, missed the whole of pre-season and the opening three months of the season due to injury, and has only made seven appearances for United this season.
"Bob’s gone to Fleetwood on loan, he needs some football. He has only played five league games for us this season, and has been out for a long, long time.
"It will be beneficial for him to go and play some league football in League One, and we have a 24 hour recall on him."