Marc Laird will be out for between six to eight weeks after damaging his medial ligaments in the game against Northampton Town on Saturday.
Laird had featured in all three of Blues’ league games so far this season but is now likely to be out until at least the start of October.
Phil Brown said: “It’s really disappointing because Marc has made an excellent start to the season and done really well.
“It’s also a worry as it means with Julian Bennett out our squad is going to be tested even further over the next few weeks.
“It may give some of the younger players the opportunity to impress.”