Bentley is ineligible as he has already played in the competition for his parent club, Tottenham, this season. Sebastian Larsson is set to replace Bentley as City look to overturn a 2-1 first leg deficit.
Blues boss Alex McLeish has no fresh injury concerns with James McFadden (knee) and Scott Dann (hamstring) long-term casualties. McLeish wants to put all off-field issues to one side for at least 24 hours as Birmingham try to reach their first major Wembley final for 55 years.
The Scot has been frustrated by his inability to bring in a new striker during the transfer window, with attempts to sign Robbie Keane and Kenny Miller ending in failure. Then Birmingham's acting chairman Peter Pannu questioned the success rate of some of McLeish's signings during the Carson Yeung era ahead of Saturday's 5-0 mauling by Manchester Old Trafford.
But McLeish has remained a dignified figure throughout and is determined nothing will distract from the clash with West Ham. He said: "We are still trying to enhance the squad. We just have to wait and see.
"If it goes to the wire, it goes to the wire, but I am trying to help the lads to improve for the second half of the season. It has been frustrating in one or two situations but I will put it to bed for 24 hours and get this semi-final out of the way.
"As for Peter's comments, we are not getting involved in those comments. We are only talking about the Carling Cup."
Gary O'Neil will go straight into the West Ham squad after completing his move from Portsmouth. Manager Avram Grant is without suspended strikers Victor Obinna and Frederic Piquionne while the club were unable to complete the loan signing of Demba Ba from Hoffenheim in time. But Carlton Cole is expected to be fit to start up front after missing the draw at Everton and on-loan defender Wayne Bridge is available because he is not cup-tied.
Grant said: "To be at Wembley after such a long time would be very good and, if I may say it, we deserve it. Our route to this game has been very good. We won the first game (against Oxford) in the last minute.
"We played very well to beat Sunderland away, we showed a lot of character against Stoke and we were brilliant against Manchester United.
"That is the only game they have lost all season. But we know that no-one will give us any presents. It will mean a lot also to Birmingham - but for us it is very important. We have a good record in the cups this season and it has also been good football. We have scored a lot of goals and we want to continue like that.
"There is one more game to the final."