The 28-year-old was on the verge of getting a deal from United until the end of the season before a back injury ended the agreement, but he has remained with the club under new boss Gary McAllister and is continuing to work on his conditioning.
Christie's career has been blighted by a string of serious injuries - including two broken legs and a fractured foot - but he turned down the chance to join Blue Square Premier club Burton Albion last month in favour of winning a contract in the Football League.
McAllister said: "He's a guy who we've offered our facilities to for him to get back into shape. Come the time when he's fit again, he's a player who's proven that he can score goals at a higher level than where we are at the moment. But he's got a lot of rehab to do and it'll be further down the line with him."