The influential centre-half was stretchered off the field in the first half at St Andrews following a challenge by Plymouth's Jamie Mackie.
Blues boss Alex McLeish, who absolved Mackie of any blame, said: "He's (Ridgewell) not good. I don't know the full report but it seems like he's bust a leg - he has a broken bone in his leg.
"I don't know to what extent, but suffice to say his season is over. I think it was his left leg, his kicking foot and stronger leg.
"I don't think the challenge was anybody's fault. It was just the way he landed because his knee seemed a bit open at the time as he played the pass."