Centre-back and lifelong Chelsea fan Johnson and left-back Ridgewell suffered knee and ankle knocks respectively during the 1-1 draw with Everton at Goodison Park on Sunday.
Johnson has been ever present in City's 18 league games since his £5million move from Cardiff during the summer while Ridgewell has impressed since being converted from a central defender into his current role.
If the duo recover, Birmingham are expected to be unchanged for the seventh successive game.
Kick-off at St Andrew's is 12.45pm.
Blues (from): Hart, Carr, Roger Johnson, Dann, Ridgewell, Larsson, Ferguson, Bowyer, McFadden, Jerome, Benitez, Maik Taylor, Fahey, Vignal, Carsley, Phillips, McSheffrey, Damien Johnson, Queudrue, Parnaby.