The Melbourne Demons pulled off a thrilling upset over the reigning premiers Brisbane Lions, securing a vital four points with a narrow 16.8 (104) to 15.12 (102) victory.

However, the hard-fought win was overshadowed by an “extremely concerning” incident with Demons defender Harrison Petty.

During the third quarter, Petty kicked to clear the ball from his own goal area but sent it straight to Brisbane player Will Ashcroft who took the mark and lined up to kick a goal.

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But 26-year-old Petty became noticeably wobbly without an obvious reason, with his teammate Blake Howes quickly running over to assist him to kneel on the field.

Despite not suffering an obvious knock beforehand, the defender had to be assisted by team doctors and taken down to the room as the game was paused.

He was promptly ruled out for the remainder of the match with what the club initially described as a “delayed concussion”.

Harrison Petty led off the field.

Harrison Petty led off the field.  Getty

Melbourne coach Steven King confirmed the club is still working to determine the exact cause of Petty’s faintness but admitted club officials suspect it stemmed from an unnoticed knock during the first half of the game.

“He was complaining of some blurred vision,” King said.

“He received that ball in the goal square and thinks ‘I probably should have stopped the game’. Whether it was anything more sinister or delayed concussion we will have a look at it.”

King said that thorough medical tests were to be conducted on Petty to establish the full extent of his injuries.

Harrison Petty of the Demons leaves the field with trainers.

Harrison Petty of the Demons leaves the field with trainers. via Getty Images

The mystery and sudden nature of Petty’s unsteadiness prompted immediate concern in the commentary booth, with Fox Footy commentators Matt Hill and Corbin Middlemas noting the deep concern from Petty’s teammates.

“In a bit of strife at the movement, teammates are actually quite concerned for him,” said Middlemas.

“So he’s kicked out and now he’s just down on one knee – so I’m not too sure what’s happened with Harrison Petty.

“Blake Howes, obviously some great teammate work there knowing something was up.

“You could read the body language, whatever he said to Howes, Howes really knew there was an issue straight away.”

The Dees prevailed over the Brisbane Lions.

The Dees prevailed over the Brisbane Lions.  Getty

“Well this is extremely concerning, he’s coming off now,” echoed Hill.

“There were four or five teammates around him straight away.”

Despite the disruption, the Demons prevailed and hung on to the important win by the slimmest of margins, now sitting above Brisbane in eighth on the AFL ladder.

They’ll look to carry that momentum when they face Richmond at the MCG on Friday afternoon.