The FA and Greater Manchester police are investigating Ben Thatcher's elbow-first challenge that hospitalised Portsmouth's Pedro Mendes during Thursday's Premier League game.
Manchester City defender Thatcher was booked after crashing into the Portuguese midfielder early in the second half of the goalless match as both players chased a ball over the touchline.
Mendes was knocked unconscious and suffered a seizure. The incident was replayed relentlessly on TV on Friday.
As Thatcher was cautioned for the offence it would normally fall outside the FA's remit for punishment by video evidence but a spokesman said that officials were investigating their own disciplinary regulations to see if there was an option for treating it as an exceptional case.
City issued a statement on Friday containing an apology from Thatcher.
"Manchester City can confirm that Ben Thatcher has personally written a letter to Pedro Mendes to apologise to him following the tackle in the match last night," it said.
"The club does not condone such action and the matter has been dealt with internally by (manager) Stuart Pearce."
The incident infuriated Portsmouth manager Harry Redknapp who said after the game:
"What do you have to do to get a red card these days, kill someone?A Greater Manchester Police spokesman said they were investigating the incident after receiving complaints.
On-pitch confrontations rarely pass beyond an initial police investigation, although last month Lee Bowyer was fined $NZ1700 plus costs by magistrates after pleading guilty to threatening behaviour after he punched former Newcastle United team mate Kieron Dyer during a match last year.
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