The promising left-back, 20, was forced off late on in last weekend's 2-2 draw at home to Middlesbrough and scans have confirmed a ligament strain.
In addition, Blues striker Lee Novak has been sidelined for the short-term with a leg problem while defender David Murphy, already on the treatment table nursing a troublesome knee complaint, has been told he will not return to action until late February at the earliest.
Manager Lee Clark said: "Mitch has taken quite a serious smash to the knee, as you can imagine with the ferocity of the tackle, so he won't be with us until the new year.
"But hopefully not too long into the new year. Possibly even the New Year's Day game at a push. To be honest, I knew straight away because he doesn't go down for nothing and then when he tried to play on, within seconds flagged it up.
"We'll also be without Lee Novak on Saturday after he took a nasty hit to his leg, he's had an X-ray and a scan to see if there's any fracture, thankfully that isn't the case. He's got an outside chance of being ready for Forest (December 21).
"And also we've had news that the best we can hope for with David Murphy will be late February to early March. So we're really at the bare bones in terms of defenders."
Clark this week recalled defenders Will Packwood and Amari'i Bell from their loan spells at Bristol Rovers and Nuneaton respectively.
The pair are both in the frame for Saturday's Championship trip to Bournemouth.
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