Defender Scott Laird was floored by the blow as fans ran on to the pitch at the final whistle as the League Two side celebrated yesterday's 3-1 win over the Toon.
At first it was thought to be a Newcastle fan ... but if you look close to the video footage he is carrying a Stevenage scarf.
Laird was celebrating on the halfway line following the game, then he is suddenly approached by the fan who punches Laird directly in the face, knocking the player to the turf.
Stevenage manager Graham Westley: "Scott Laird was stood celebrating with the fans and a fan just came over and smashed him from the side.
"He went to the ground and it is a shame for the afternoon to end in that way.
"He is a bit bewildered. He (the fan) gave him a right-hander, just smashed him across the cheek and knocked him to the ground.
"He said to me, 'It's lucky I've got a big head to take it'. It was a sad way for things to end."
An FA spokesman said: "It's something that should be flagged up in our meeting on Monday.
"It might be that there was a stadium adviser at the game who'll be able to advise us on that."
Police have described the fan as a white male wearing a white Diesel top and red and white Stevenage scarf.
LOOK AT THE FOOTAGE SENT TO US BY A STEVENAGE FAN FROM HIS MOBILE PHONE:
http://www.youtube.com/watch?v=dlvVgkXA_r0
Watch from 1:36, as he jumps from the Stevenage fans, wearing similar clothing to the clip showing the punch and waving a scarf in right hand.
THE FULL EVENT: