"I think Jon performs to his best when he is happy in his environment and I think it is true to say that he hasn't been over the last few years," Warnock told the Blades official website.
"That's what he needs right now and I am expecting him to complement our left side which is a strong part of the team.
"He's also a smashing lad in the dressing room and that is another reason why he will be an important signing."