The Northern Ireland international midfielder has been forced to sit-out the first three games of the season for a sending off in Blues' last away game of last season against Norwich City.
Johnson was dismissed for violent conduct in the game at Carrow Road which carries an automatic three game ban but as it was his second dismissal of the season he was suspended for four games and so sat out the last home game of last season against Arsenal as well.
However, the suspension has now been completed and Johnson is available for selection for the game at the Hawthorns on Saturday lunchtime.