Johnson sustained a knee injury during the away draw at Everton on Sunday whilst Ridgewell finished the match at Goodison Park with a calf strain.
Both players have been unable to train so far this week but will resume training on Thursday and should both be fit in time for the Chelsea match at St Andrews.
"They have not trained so far but should join in on Thursday. We are probably looking to field an unchanged side against Chelsea," manager Alex McLeish told Sky Sports
"Roger has been magnificent since coming here from Cardiff and Liam has looked a natural since moving from centre-back to left-back."