Police have arrested a man after Sheffield United captain Billy Sharp was allegedly assaulted by a fan at the end of their Championship play-off semi-final second leg at Nottingham Forest.
Television footage showed a supporter charge at Sharp during a pitch invasion as the player stood on the sidelines.
Forest won 3-2 on penalties after the two legs finished 3-3 on aggregate.
Sharp, 36, said "one mindless idiot ruined what was an unbelievable night of football".
In his post on Twitter, he added: "Congratulations to Nottingham Forest on their victory and good luck in the final. As an ex-Forest player I will not let one scumbag ruin my respect for the Forest fans.
"Incredibly proud to captain this group of Sheffield United players, they gave their all and can hold their heads up high."
Blades manager Paul Heckingbottom said Sharp required stitches, while Forest say they were "appalled".
"We've seen one of our players attacked," said Heckingbottom after the match at Forest's City Ground. "He's shook up, bleeding, angry.
"Bill was minding his own business off the pitch, trying to get our players away. He was blindsided, it was cowardly. He was knocked to the ground [and required] stitches, he was shook up."
The FA said: "We are aware of the very serious incident that took place after the match between Nottingham Forest and Sheffield United.
"We will be investigating the incident as a matter of urgency and will be working alongside the clubs and the relevant authorities to ensure the appropriate steps are taken."
Nottinghamshire police launched an investigation following the incident.
Chief Inspector Paul Hennessy, of Nottinghamshire Police, said: "A 31-year-old man has been arrested on suspicion of assault in connection with this incident. He remains in custody and will be questioned by officers.
"Our enquiries into the circumstances surrounding the incident are ongoing. We have spoken to the player concerned and we are working with both clubs who are supporting us with our investigation."
Sharp who did not feature in the game because of an injury, played for Forest on loan in the 2012-13 season.
In a statement, Forest said the offending fan would be banned for life.
They said: "Nottingham Forest Football Club are appalled to learn that our former player, Billy Sharp, was assaulted leaving the pitch after tonight's match at The City Ground.
"The club will work with the authorities to locate the individual in order they are held to account for their actions, which will include a life ban from Nottingham Forest.
"The club would also like to apologise to Billy personally and to Sheffield United Football Club."
After the game, Heckingbottom told Sky Sports: "How can you come to your place of work and mind your own business and be assaulted in that fashion. It's bang out of order and everything that's wrong.
"You hear too many times that people condemn it, but nothing is done and we are the ones who suffer. It is upsetting for many reasons but it will be dealt with."
Sky Sports pundit and former Forest defender Michael Dawson called it an "absolute disgrace."
In January, the UK's football policing lead told the BBC that arrests at football matches across the top five English leagues were at their highest levels in years, with fan disorder "getting worse".