Tony Capaldi, a former Blues left-back and Cardiff City team-mate of Johnson's, told the Birmingham Mail: "Roger is a confident lad, a strong character and an excellent defender.
"He's been the best centre-half in the Championship and our player-of-the-season for the past two years.
"He's definitely captain material. He's that sort of leader.
"He's in his mid-20s now, he came through when he was very young at Wycombe Wanderers and probably has almost 300 league games under his belt, so he's got the experience and is now ready to step up.
"Whether he will be able to, we're about to find out, but I'm sure he will be fine and he will be a success at Blues."