Midfielder Larsson did not travel with the Blues squad for the match against QPR on Tuesday night having picked up a groin strain, Mehdi Nafti was named in the side in place of the injured Swedish international.
Striker O'Connor had been expected to play against Rangers but he sustained a groin injury during the pre-match warm-up and was replaced in the starting eleven by James McFadden.
Defender Parnaby did start the game at Lotus Road but was strechered off and substituted by Jared Wilson who had received a late call-up to the 16-man squad following O'Connor's withdrawal.
The Blues right back sustained a hamstring injury 12 minutes into the 1-0 defeat in West London which if proves to be serious could mean he will be sidelined for at least three to four weeks, maybe longer!