Blues left-back Nathan Ralph dislocated his shoulder in Tuesday night’s draw with Burton Albion.
Sol Campbell confirmed the news ahead of Saturday’s game against Bristol Rovers.
Campbell said: "It’s not the best news for Nathan, he’s dislocated his shoulder.
"It popped back in but they’ve got to assess it now and he will have a scan to see how much damage has been done.
"That will give us a timescale but I’ve done it before and it’s three months really.
"If it’s not too bad it might be a bit less but it’s a shame for us because it was all quite innocuous.
"He pushed his arm up and it’s just popped out so it was unlucky.”
When asked if Elvis Bwomono or Jason Demetriou were the best options to deputise, the Blues boss added: "Not really, no.
"There’s other options. I think Elvis can stay where he is, and we can find another solution.
"You have a squad and you have to utilise the whole squad.”
Brandon Goodship came off with a groin injury on Tuesday night but should be fit, along with Stephen McLaughlin who took a whack to the face in the 1-1 draw with the Brewers.
"He was stretching his groin a little bit and he did the right thing to come off so he should be alright for the weekend,” Campbell said on Goodship’s availability.
Ethan Hamilton, meanwhile, will return to Blues on Friday having gone back to Manchester United for treatment on a groin injury and will be assessed ahead of Saturday's game against Bristol Rovers.