He damaged his Anterior Cruciate Ligament while on loan at Brighton at the end of last season but has recovered well and could be available after the international break.
Assistant Mick Jones is delighted.
"We have just received the all clear from Calvin's surgeon for him to start playing again," he said. "It really has been a remarkable come back because he is way ahead of schedule.
"The surgeon has done a magnificent job on it, but Calvin has worked so hard during his rehabilitation. I am absolutely delighted for him. The injury was potentially career threatening but I have never seen a player work as hard as him.
"Hopefully by the time the international break is over we will have another player to choose from."