The former Cardiff player was expected to be back available within two to three weeks but it will now be early October before he is ready to return.
Blues manager Alex McLeish confirmed: "Jerome has had scans on the troubled area and the news is not great.
"It's enough to keep him out for maybe another month. It was one of those ligament-type injuries.
"He got a kick on his ankle and heel. It's a blow as Cameron was looking to show us that he's maturing into a Premier League player."
Meanwhile, midfielder Sebastian Larsson admits his hopes of signing a new contract may have to be shelved until the takeover of the club is completed by Carson Yeung.
The Sweden international had opened talks with City over a five-year deal.
Larsson admitted: "It's gone kind of quiet since the situation with the sale of the club. I don't know whether anything can be done now (with a new contract).
"We were in decent stages of the contract talks but, since all this happened, the club haven't come back with anything.
"But let's not worry about that because I still have time left on my contract and, until the club come back to me and say they want to do business, I am not going to let it bother me."