West Brom's Chris Brunt was hit by a coin thrown by Albion fans after their 3-1 defeat at Reading and said he was ashamed of the Baggies' support.
The Football Association confirmed it is investigating the incident, while police are treating it as an assault.
The Northern Ireland international had gone over to applaud the visiting fans after their FA Cup fifth-round exit but went down clutching his face as he approached the away end.
"I've still got the 50p. I spotted a little girl in the front row and wanted to give her my shirt," Brunt said. "Obviously there's some people in the front who were angry, understandably, because we didn't play well or got the result we wanted.
"But there's absolutely no excuse for that and no place for it in football. I've been here a long time. Obviously we all get our fair share of stick but to be hit with a coin by your own fans is absolutely disgusting.
"The guy who threw it knows who he is and knows all this week where I'll be, and next Saturday if he wants to come and have a word about it that's not a problem -- I'll meet him wherever he wants.
"I've been here a long time, and I've loved my time here and that's the first time I've been ashamed of our supporters. A small minority obviously ruined it for the rest of the 4,000 who were there and paid good money for a day trip.
Chris Brunt goes to give his shirt to a young girl in the WBA end and gets coined by his own fans. pic.twitter.com/6f4CgVZHTu
- Michael Regan (@MichaelRegan) February 20, 2016
The FA will look into the incident, as well as the crowd disturbances in the Baggies' end of the Madejski Stadium.
A statement from the FA on Saturday evening read: "The FA is investigating the incident in which an object was thrown from the crowd at Chris Brunt at the end of the Reading v West Bromwich Albion game on Saturday (20 February 2016) as well as the crowd disturbances in the away section of the stadium.
"We strongly condemn any such behaviour and will work with both clubs and the authorities to identify those responsible."
A Thames Valley Police spokesperson said: "The incident will be dealt with as an assault and we will be working with West Bromwich Albion and Reading to resolve the issue and identify anyone involved."
Press Association Sport reported the fan who allegedly threw the coin has been identified by stewards in a picture that has now been passed to police, and West Brom will issue a lifetime ban if the person is found guilty.
A club spokesman said: "The offender can expect the severest possible sanctions." TV pundits also condemned the action and echoed Brunt's comments.
West Brom manager Tony Pulis said the player could have been blinded.
"We have seen an incident where a lad who has played over 300 games for this club could have had his eye out," Pulis said.
"He has a cut and it's not too far away from his eye. It's an inch away from striking him in his eye, and if that happens we're talking [about] a very, very serious problem.
"It's barbaric. That's not acceptable. West Bromwich Albion is a fantastic football club, a famous club, but you can't condone that.
"I think the police have to get involved in this. I don't think you can allow that to go by. That's criminal, and I mean criminal.
"This has got to be one of the worst things I have seen, for Chris to have a coin thrown at him from his own supporters having been at the club for such a long time. Every decent supporter of the club will be absolutely devastated it has happened to their football club.
"It's the way the game is going. Social media and everything else builds up an anti-feeling if things are not going well at football clubs. We have to be very careful with it."
Brunt, 31, did hand his shirt over to a supporter after gesturing and remonstrating with another group that had made its way to the front of the stand as he moved closer to apologise for Albion's exit.
Reading boss Brian McDermott added: "It's outrageous, stuff like that -- it's not right and proper. We will co-operate with whatever we have to and make sure we find the person who threw it."