Blackwell said: "Rob's going to be out for a couple of weeks. We'll have it scanned when we get back to Leeds but for the meantime he's had it strapped up and immobilised.
"We now have to work on getting Rob right for the games at the end of the month and hope he can build up his match fitness in those," he added on the club's official website.
Meanwhile, goalkeeper Neil Sullivan has been told he can return to full training next week after undergoing surgery on his shoulder and Steve Stone is making progress after tearing his Achilles.