Porter took to the field in the 87th minute with the scores tied and walked off having netted twice to secure a stunning 3-1 win for the Sky Bet League One side.
Blades boss Nigel Clough found out at half-time that the winners at Bramall Lane faced a home tie against either Sheffield Wednesday or Charlton and used the possibility of a derby date with the Owls 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 when the news was revealed.
"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 per cent 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 was 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 cup scalp - with Aston Villa and Fulham already vanquished in earlier rounds - went down well with home fans, though a mini pitch invasion at full-time may have crossed the line.
"There were 25,000 in today and the fans were incredible, that's the sort of atmosphere you want to play in," said Clough.
"The only downside is a few supporters coming on the pitch. You understand the exuberance, but just try and stay off the pitch."
Forest had taken the lead through Jamie Paterson's 28th-minute header and held it for 40 minutes until goalkeeper Dorus De Vries gifted Conor Coady the simplest of equalisers.
Although Clough admitted using the quarter-final draw to lift his players at half-time, Forest boss Billy Davies did not take issue with the timing.
"Late kick-offs, early kick-offs, draws before the game, draws after the game...I've got enough to worry about without trying to disagree with the powers that be," he said.
"I've no issues at all. We accept when the draw takes place so I have no problem with it."
He did, though, have a problem with his players' efforts after the break.
"The first 45 minutes we had total domination, the second half is complete and utter self-destruct," he said.
"Looking at their goals, it is individual error. We gave them a huge lift and by the end they looked much hungrier."
Davies and Clough endured strained relations when the latter managed Derby, but the Scot confirmed they had shaken hands prior to the game.