Sunderland defender Luke O'Nien was "punched" by a Portsmouth supporter during Thursday's League One play-off semi-final second leg at Fratton Park, according to team-mate Lee Cattermole.
The incident, which happened after O'Nien tumbled over the advertising hoardings chasing the ball, led to a fan being escorted from the stand.
After the game, O'Nien played it down as "just a bit of handbags".
Sunderland sealed a 1-0 aggregate win to reach the final at Wembley.
"It's nothing to think about too much," added O'Nien. "It's happened and it's very silly.
"I've seen my little cousin hit harder than that, so not a problem with that."
He said he was asked by police if he wanted to press charges, but declined.
Sunderland manager Jack Ross said the incident during the goalless draw "wasn't great" but that he "wouldn't make a big deal of it".
However, Cattermole told BBC Radio 5 Live: "He was punched in the back - I was very disappointed and very surprised that people think they can do that when they come to watch a game.
"Someone like Luke as well, he's a fantastic lad. He doesn't need or deserve that.
"It was a tense atmosphere, I've played here a few times. It's a great ground with a cracking support and it's a shame sometimes one lets them down."