Steve Schumacher has apologised for swearing at City fans.
The midfielder has been fined by the club after the outburst, which happened during the Chesterfield game.
"I should be more professional in those situations," admitted Schumacher. "I apologise to anybody who was offended by my language, particularly any children, and I promise it won't happen again.
"It was down to frustration and anger at what was going on. But that's no excuse and I'm sorry if I upset anyone."
Caretaker-boss David Wetherall said: "We have looked into it and he has been fined. We are all together, players and fans alike, and he has been disciplined."
Meanwhile, Eddie Johnson will sit out the final game of the season because of a groin injury.
Johnson has got a problem with his abductor muscle and, with their relegation fate already confirmed, City do not want to risk making the injury worse against Millwall.
Wetherall added: "I should know about this. It's exactly the same place where I had trouble and I played with it for a long time until it snapped at Derby.
"We're not worried about the same thing happening with Eddie and there is no reason to think it should but he has had this a couple of weeks and, although it is nothing serious, there is no point in risking him."
Johnson's form has been a revelation since being switched to central midfield and he earned the T&A/Bradford & Bingley player of the month award for April.
Wetherall admitted his consistent performances have left City with a dilemma.
"Eddie has taken to the position like a duck to water and we've been really pleased with him. The question now is whether he is still seen as a striker or a midfielder?
"He has enjoyed playing the role and that is a big part of it."
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