United fan Tyler Machin stepped up after a call went out at Brammal Lane for qualified match officials when the assistant referee was substituted at half-time due to illness.
Luckily the plucky teen was already dressed in his full ref kit - having travelled straight to the stadium after refereeing a league game in Worksop in the morning.
Tyler, who described the FA Cup match day experience as “amazing”, became fourth official during the game’s second half.
He told the BBC: “Half time over the tannoy an announcement that the assistant referee had fallen ill.
“Can we have a replacement from the crowd - is anybody qualified in the crowd?
“So I thought, yep, here’s my chance.
“And it’s always been an ambition of mine to get into a professional game in terms of refereeing.
“So to do it at Sheffield United was extra special.”
Tyler’s dad found out about his son’s guest appearance later while watching Match Of The Day.
The teen told how he waited until he appeared on the screen before saying to his dad ‘oh look, do you recognise him on the telly?’
Other family members and friends found out through social media as he appeared at the side of the pitch.
Following his miraculous appearance on the touchline Tyler took to Twitter.
He wrote (SIC): “Damm, I make a TV appearance and what happens.....dead trim.
“Wish I'd done down to the the barbers the other day to get that skin fade back in.”
As Tyler stepped forward during the game AFC Fylde wrote from their official Twitter account (SIC): “There's just been an announcement for a qualified referee to come forward, and someone has just turned up in full kit!
“Talk about seizing the opportunity.”
@kjfairhurst89 wrote (SIC): “Surely you wouldn’t wana do that full time @tyler_machin you’d miss being at back of kop with us”.