Doncaster Rovers right back Jamie Sterry was spotted playing with a huge lump on his head against Crystal Palace in the two sides' FA Cup game on Monday.
The former Newcastle player started the game at the Eco-Power Stadium, but was taken off in the 80th minute after he had sustained the injury.
The League Two side lost 2-0 to Premier League club Palace in the fourth round of the FA Cup on the night with goals from Daniel Munoz and Justin Devenny, and will host Millwall in the fifth round.
However, leaving the pitch in the late stages of the match, the 29-year-old could be seen with a golf ball-sized lump on his head.
When manager Grant McCann subbed off the Englishman, he appeared in good spirits despite the injury and the fact his side were 2-0 down.
It is understood that the player received medical help after leaving the field.
Sterry has had a long career in England but is mostly remembered for his time with Newcastle, his boyhood club, where he featured between 2015 and 2019 and also included two loan spells at Coventry City and Crewe Alexandra.
He was part of the Magpies side that won promotion to the Premier League in 2017 with manager Rafael Benitez on board after a year in the Championship.
Newcastle have seen significant progress since following a takeover, while Sterry has settled after his move to Doncaster in July 2023 in a free transfer from Hartlepool United.
However, his side lost to Palace, who heavily rotated their team, on Monday. Chelsea loanee Ben Chilwell appeared in the second half to play his first game since October.
In the fifth round of the FA Cup, Oliver Glasner's men will host south London rivals Millwall, who beat Leeds United on Saturday.