Muscat was dismissed for a reckless stamp on Watford striker Danny Webber during Millwall's 3-1 defeat at Vicarage Road.
The Australian international was making just his second appearance for The Lions since his free transfer switch from Rangers last month.
But the 30-year-old is already treading on thin ace at The New Den after being told any repeat of last Saturday's incident will terminate his Millwall career.
"Kevin's actions at Watford were totally unacceptable and we have made him aware that such behaviour will not be tolerated by this football club," chief executive Ken Brown told the club's website.
"Kevin has been issued with a final written warning, confirming that any repeat of such a deliberate, unprovoked, off-the-ball incident will result in his instant dismissal.
"He has also accepted a fine in excess of the contractual maximum of two weeks, acknowledging the serious nature of his actions.
"Kevin now wishes to put this matter behind him and has pledged that his future conduct, both on and off the field, will demonstrate his responsibilities as a professional footballer with Millwall Football Club.
"As far as the club is concerned, this matter is now closed."
