Bentley is ineligible for the tie as he has already played in the competition for his parent club, Tottenham, this season.
Sebastian Larsson is set to replace Bentley as Blues look to overturn a 2-1 first-leg deficit.
Manager Alex McLeish has no fresh injury concerns, with James McFadden (knee) and Scott Dann (hamstring) still long-term casualties.
Source: PA
Source: PA