McFadden has suffered a reaction to the knee problem which required keyhole surgery five weeks ago although Blues manager Alex McLeish is hopeful he can still play.
Ridgewell had to be substituted at half-time during the 2-0 defeat against Fulham at Craven Cottage with an ankle injury.
City need to win and then hope that Reading and Fulham slip up if they are to maintain their top-flight status.
Striker Benni McCarthy is set to return to Blackburn's squad.
The South Africa international, who has scored 11 goals this season, has missed the last two matches with a hamstring injury but has trained fully this week.
David Dunn (groin) could also get his place back in the squad against his former club while Zurab Khizanishvili should be fit despite a minor groin problem which caused him to miss a day's training.
Teams:
Birmingham (from): Taylor, Doyle, Kelly, Ridgewell, Jaidi, Schmitz, Murphy, Parnaby, Larsson, Johnson, Nafti, Muamba, McSheffrey, Kapo, Forssell, McFadden, Jerome, O'Connor.
Blackburn (from): Friedel, Reid, Nelsen, Samba, Ooijer, Warnock, Bentley, Emerton, Vogel, Pedersen, Santa Cruz, Roberts, Dunn, Brown, Tugay, Derbyshire, McCarthy, Mokoena, Khizanishvili.