Charnock, 24, moved to London Road from Northwich Victoria last summer but has made only 15 appearances for the club.
His only game this season was the 1-0 League One defeat at Scunthorpe, where his clumsy challenge on Williams gave United a penalty. Paul Hayes converted it to give the Iron a much-needed first win of the season.
Ferguson singled out Charnock for criticism after the game, and his name has been circulated to all Football League and Blue Square Premier League clubs.