Sunderland boss Lee Johnson has controversially suggested keeper Lee Burge's minor heart trouble could have been caused by a Covid vaccine.
The shot-stopper has been ruled out for five weeks to allow an "inflammed heart" to be treated.
Burge has been instructed not to let his heart rate go above 100 during gentle training after a bout of Covid.
The British Heart Foundation have reported rare cases of inflammation of the heart, known as myocarditis following Moderna, or more rarely, Pfizer vaccines.
Cases have usually been "mild" and in "most cases heart function returns to normal" after rest and treatment.
Getting Covid can also cause myocarditis as well as other severe illness.
Johnson said: "It seems to happen a lot after these injections or Covid.
"The weighting would be on Covid but you can't rule out, I suppose, vaccination.
"I don't know, I am not a doctor. I get the information and the first thought is the player. Is he fit and well and survives it well and has a long career."
Challenged on his vaccine claim, Johnson added: "That's probably slightly controversial. I don't know the answer. I would be ill advised to make quotes like that.
"Burge was not particularly happy the way he felt during that period. That could be a cross over when he had Covid and when he had the vaccine. We don't exactly know what caused it, it is probably the Covid.
"He has 4-5 weeks when he has to keep his heart rate below 100bpm and have blood tests and hope it goes back to normal.
"The care of the individual has to be at the forefront. Respect goes to the physio and dr department. We don't want something dreadful happening. Any sign of heart fluttering or not feeling right and we have the right checks and it is flagged up. He should make a full recovery."
Football authorities have been battling to encourage players to get vaccinated, with games being postponed, especially over Christmas and New Year, because of outbreaks.
A statement from the EFL in December read said: "75 per cent of players across the EFL are either fully vaccinated, have had a single jab or intend to be vaccinated.
"Double vaccinated players total 59 per cent, 16 per cent are set to get the jab, while 25 per cent of players currently do not intend to get a vaccine."
The British Heart Foundation website adds: "Because vaccine-associated myocarditis is a recent and rare issue there is uncertainty about its long-term health risks."
As of Thursday, there have been 52,281,873 first vaccine doses administered in the UK - around 91 per cent of people aged over 12.
The Intensive Care National Audit and Research Centre reported on 31 December that 61 per cent of patients admitted to an ICU in December were unvaccinated - which included people with only one jab.