Ben Marshall has apologised for clashing with Leicester supporters during the Easter Monday defeat at Barnsley.
The midfielder was involved in an altercation with some travelling fans during the second half of a 2-0 defeat at Oakwell.
The result left City two points outside the play-off places with just one win in 11 Championship games.
Marshall told the club website: "In the heat of the game, I lost my composure and reacted in a way that was unacceptable, which I truly regret.
"I let myself, my team mates and the supporters down and unreservedly apologise for any offence I may have caused.
"We've set high standards as a squad this season and we share the frustrations of the fans that, as a team, we haven't performed as well as we should have in recent weeks.
"We're working hard to put that right in our last six games, starting at Brighton on Saturday, but I realise that we have to stick together as a club to make it happen - players, staff and supporters.
"The fans have been amazing this season, both at home and away, and we're going to need them over the coming games so I hope they stay behind us."