The 28-year-old has been training with United for the last few months and has now signed a three-month deal at Bramall Lane.
“We have not paid Jonathan a penny while he has been training with us and he has worked his socks off,” said Blades boss Kevin Blackwell.
“When he first arrived, he was nowhere near the levels where he should be, he couldn’t turn or twist and there was no burst of speed.
“But he has shown a true professional’s attitude and we know he is getting close now.
“He is still only 28 and when he performs he is a Premier League or top Championship player.”
