The FA are investigating an incident in the First Division play-off semi-final 1st leg against Millwall at St. Andrew's.
The assessor at the game has reported Horsfield after he allegedly kicked out at an opponent in an off-the-ball incident.
If he is found guilty he could face a three-game ban for violent conduct which would mean him missing Blues' opening games against Arsenal (a), Blackburn (h) and Everton (a).
