Porter took to the field in the 87th minute with the scores tied but walked off having scored twice to secure a 3-1 victory for Sheffield United.
Nigel Clough, his manager, found out at half-time that the winners at Bramall Lane faced a home tie against either Sheffield Wednesday or Charlton and used the excitement of a derby date to inspire his side to overturn a 1-0 deficit. But Porter, who nervelessly converted a last-minute penalty before bundling home a second in injury time, was on the pitch with his fellow substitutes.
"I was out warming up [when the draw was made] so I wasn't aware who we might be playing," said the striker. "A fan shouted on about it but I didn't know whether he was serious so I wasn't 100% aware who we'd be playing. We didn't need any extra motivation than playing for the quarter-final of the FA Cup but it would be massive. It would be jam-packed here for Wednesday, people trying to get in all over the place."
Porter's ignorance of United's potential opponents may have helped when he took the decisive spot kick, but Clough had been happy to let the rest of his side know. "Yes, we told them just before they went back out," he said. "We thought 'what the hell?', we'll just tell them. I just said: 'you've got 45 minutes, away you go. If we get the victory you might have a Sheffield derby yet'. A few of them didn't believe us actually so we just joked and said it was Brighton or Hull away.
"We had to come back and win the match so it might have been a small incentive for the players."
United's latest scalp - with Aston Villa and Fulham already vanquished - went down well with home fans, though a mini-pitch invasion at full-time may have crossed the line.
"The fans were incredible, that's the sort of atmosphere you want to play in," said Clough. "You understand the exuberance, but just try and stay off the pitch."