He was that dirty.
In fact, many consider the Australian to be the filthiest footballer ever to 'grace' the game.
His 19-year playing career, which saw spells at Crystal Palace, Wolves, Rangers and Millwall, made Roy Keane look like an angel.
But all of that has changed - and a 48-year-old Muscat now finds himself in the same progressive football family as Pep Guardiola.
To understand how mad that is, let's run through some of the former defender's career highlights.
Where to begin? Ian Wright, during his Nottingham Forest days, called Muscat a 'nobody' and a 'lowlife' after hearing him shout 'LEAVE IT' while playing for the other team. A classic gag.
But some of Muscat's victims came off far worse. Just ask Matty Holmes.
He faced initial fears his leg would be amputated following a tackle from the Aussie in 1998, eventually winning £750,000 in a court settlement after four operations salvaged the limb.
That same year, Muscat left Craig Bellamy with a puncture wound in his kneecap, while Chistophe Dugarry was ruled out for three months with ligament damage after an ugly challenge from the right-back.
It happened during a 'friendly' between Australia and France, but Muscat didn't know the meaning of the word, and his tackle was described as an 'act of brutality' by French manager Roger Lemerre.
Incredibly, there have been worse things said about Muscat. Ex-Birmingham player Martin Grainger described him as 'the most hated man in football' after a reckless tackle on his own Australia teammate Stan Lazaridis.
Muscat's aggression sparked countless brawls, earned him numerous suspensions, and was too much for Millwall.
Three weeks after joining the Lions, Muscat was fined by the club after giving away a penalty and getting sent off for stamping on Watford's Danny Webber, and chief executive Ken Brown warned he would be sacked unless he changed his ways.
In that same match against the Hornets, an 18-year-old Ashley Young, making his senior debut, later revealed he was threatened by Muscat before kick-off.
"Muscat said he would break my legs if I went past him," Young said in 2010. Ironically, the defender was sent off before the winger came on as a substitute.
Even Neil Warnock was appalled by Muscat after he sparked a mass brawl between Millwall and Sheffield United in 2004.
The Blades went on to win the game and Warnock was seen telling opposition players, 'serves you right for f****** Muscat, that'.
His wild time in England came to an end in 2005, but the antics continued.
Muscat quickly became the first A-League player to appear before a FFA disciplinary hearing just months after joining Melbourne Victory.
Coaching staff were not safe from Muscat either, and in 2006, he was sent off for pushing Adelaide United manager John Kosmina off his chair while retrieving a ball.
His playing career ended in the most fitting way possible: with a season-ending eight-week ban for a dirty tackle in the Melbourne derby in 2011.
The decision to retire came from being unable to keep up with the pace of the game, but in truth, Muscat never arrived at the ball on time.
You get the picture: when it came to the player, it was like watching a lovechild of Lee Cattermole and Vinnie Jones.
But Muscat the manager? Well, that's an entirely different story.
The uncultured defender, who tops every list of football's hardest ever men, is now a forward-thinking coach guiding Yokohama FM to the top of Japan's J-League.
"I want to be involved in teams that play a certain brand of football and that maybe contradicts what most people perceive to my playing days," Muscat recently told AFP.
"I'm trying to forge my own way in terms of the way I coach, the way I manage and the way my teams play.
"That's really all I can do."
Having succeeded Celtic manager Ange Postecoglu in 2021, Muscat guided Yokohama FM to a second-place finish last season and now his side are top of the table after 16 games.
Yokohama FM are one of the lesser-known members of the City Football Group, the holding company which owns Manchester City, Melbourne City and New York City.
And after 19 years of crunching tackles, it seems Muscat has finally discovered the beautiful game, just like Guardiola at Yokohama's famous sister club.