Mark Molesely has confirmed Nathan Ralph will be missing for Blues this weekend against Tranmere Rovers.
The left-back was stretchered off 10 minutes into Tuesday’s defeat away at Salford City.
The extent of the damage will be established next week when Ralph visits a specialist.
"I’ve just heard he’s having a scan and seeing a specialist at the same time,” said Molesley.
"We were going to scan him at the earliest opportunity, but at the start of next week he’s going to simultaneously have a scan and see the specialist.
"There’s some swelling in the knee so we thought that was the most prudent way to go and I can’t report anymore at this stage."
Molesley called it a ‘bitter blow’ to be without Ralph but will look at who will come into replace him.
"It’s another tough one to take,” he added. "Nathan has been absolutely fantastic ever since we walked through the door. A fantastic player and a great person to have around as well. I can’t speak more highly about Nathan and how important he is to us but at the same time it opens an opportunity for someone else now.
"It’s a real bitter blow but we’re still waiting on the results of the scan and specialist and it’s out of our control now."
Richard Taylor replaced Ralph on Tuesday. Also waiting for an opportunity and another is Tom Clifford, and Molesley said both offer something different.
"There’s no edge to any player and we take each game on its merits.
"We know what a good player Tom Clifford is. Very technical, very good on the ball.
"Richard Taylor we felt gave us something aerially [against Salford]. Nathan Ralph is such a big player for us, a fantastic defender but he’s great in both boxes attacking and defending set-pieces, so we felt that we probably needed Richard Taylor’s aerial presence to deal with their set plays.
"We’ll take each game on its merits and pick the right team for that particular occasion."