Nottingham Forest's Championship play-off semi-final win over Sheffield United on Tuesday was marred by an attack on Billy Sharp following the final whistle.
Forest, who held a 2-1 lead from the first leg away from home, were beaten by the same scoreline back at The City Ground but advanced to a Wembley showpiece on penalties.
The two-time European champions are bidding to return to the Premier League for the first time in 23 years and will face Huddersfield Town in the May 29 final.
However, the pitch invasion that followed Forest's dramatic victory saw Sharp – a former Forest loanee and the Sheffield United captain – shockingly assaulted.
Video footage appears to show Sharp stood at the side of the pitch when a supporter ran directly towards him, flattening the 36-year-old striker with a headbutt.
Nottinghamshire Police confirmed a 31-year-old man had been arrested on suspicion of assault. He remains in custody ahead of questioning while enquiries continue.
"We have spoken to the player concerned, and we are working with both clubs who are supporting us with our investigation," chief inspector Paul Hennessy added.
A Forest statement late on Tuesday read: "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."
A furious Paul Heckingbottom, the Sheffield United manager, said: "We watch every TV game and there are always things thrown on the pitch, fan invasions, and the commentators, especially the ex-players, are always talking about the safety of the players, but nothing's ever been done.
"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 needed] stitches, he was shook up.
"How can our safety be put at risk like that in our place of work?"
Club Statement— Nottingham Forest FC (@NFFC) May 17, 2022