St Kilda defender Dylan Roberton has spent the night in hospital after his frightening collapse during his team's AFL loss to Geelong.
Roberton had lined up for a centre bounce in the second term on Sunday at Kardinia Park when he suddenly fell to the ground.
Concerned teammates and Saints medical staff quickly went to the 26-year-old and he was able to walk off the ground.
St Kilda football manager Simon Lethlean said further tests were being carried out at Geelong's Epworth Hospital after the initial diagnosis of an irregular heartbeat.
Roberton does not have a history of heart issues, Lethlean said.
"I think the doctors described it as arrhythmia … but that's just the on-the-spot diagnosis from what Dylan said himself — quite faint and dizzy and just blacked out," Lethlean told Melbourne radio RSN.
"He then had some tests and there was irregularity with his pulse.
"But obviously, the vision was quite distressing and [it's] a serious incident to make sure he's covered off properly so he's having a number of tests.
"We're taking it quite seriously, as you'd imagine.
Lethlean said Roberton's teammates were distressed about the uncertainty over his wellbeing.
"With breakages and knees, you probably know what's ahead," Lethlean said.
"I think for this one, it's probably a different feeling for players who saw it, or didn't. To see a guy collapse is a far different scenario than a collision or something.
"So I think it probably quite rightly shook a few of the players up and they were really keen to understand Dylan's whereabouts and health."
Meanwhile, Geelong forward Cory Gregson suffered another foot injury during the win over the Saints.
The Cats are worried, given Gregson missed all last season because of an injury on the same left foot.
"He's a bit sore but he doesn't know and [the doctors] don't know so we'll just have to wait," Cats coach Chris Scott said.
"Sorry to be so vague on it but I think everyone sits back on their seat and is a bit concerned, and fair enough, so we should be."
Hawks set to lose Rioli, Puopolo
Hawthorn are set to lose Cyril Rioli (knee) and Paul Puopolo (hamstring) for several weeks after they were hurt in Sunday's big win over Melbourne.
Carlton is awaiting word on the severity of captain Marc Murphy's left-foot injury.
Murphy pulled out of the side only moments before the first bounce of Saturday night's big loss to North Melbourne in Hobart.
He felt discomfort in his foot during the warm-up.