MARC Bircham has been fined £1,000 by the Football Association.
Bircham admitted a charge of improper conduct following his involvement in a touchline row with Blackburn Rovers coach Craig Short.
Short has been hit with a £2,000 penalty.
It is understood that Bircham, Ian Holloway's assistant, was handed a lesser punishment because he remained in Millwall's technical area during the dispute.
Holloway was given a formal warning by the FA after he was sent to the stands.