Clarke has missed City's last three games, including the weekend defeat Swindon, after recently picking up a calf problem.
McCall says the latest scan on the injury has confirmed a minor tear in the muscle, with the forward now facing a battle to be fit for City's next league game at home to Charlton Athletic in two weeks' time.
The Bantams chief said: "He got the knock in the Accrington FA Cup game, a bleed on the calf, but was then able to play in the Rochdale match the week after.
"Since then, we've had him scanned again and it has revealed a little tear in his calf.
"Maybe that has had something to do with the way he has been running or trying to protect it recently but there is a small tear there.
"Whether he will be fit for our next league game in two weeks, we're not too sure at the moment."