The Blues striker picked up a hamstring injury and was substituted just 18 minutes into the 1-0 win against The Saints at St Andrews on Saturday.
Manager Alex McLeish has revealed that he is still unsure as to the severity of the injury but he is expecting to be without Bent for at least three weeks, possibly longer.
"He has nicked a hamstring so whether it's the smallest of tears or not, I would have thought he would be missing for three weeks at least," the Blues boss has said.