The Belgian international struck twice in as many minutes as we hauled ourselves back from a 4-1 aggregate deficit to 4-3 with 10 minutes remaining at the Estadio da Luz on Thursday night.
Trailing 3-1 from last week's first leg at the Lane, Benfica appeared to have ended all hopes of a recovery by taking the lead through Ezequiel Garay.
We had created chances though and finally drew level on the night via Nacer's fine strike from 22 yards in the 78th minute.
Less than 120 seconds later, Nacer pounced on Harry Kane's header to score from close range to make it 2-1 on the night, 4-3 on aggregate - leaving us with one more goal to find to take the game into extra time.
We threw everything at Benfica after that. Aaron Lennon broke into the box and with goalkeeper Jan Oblak committed, Benfica were lucky to clear. Miralem Sulejmani then looked to have bundled over Harry in the box, but appeals were waved away.
The best chance saw Gylfi Sigurdsson's flying header saved at full stretch by Oblak and then Harry stretched to meet Nacer's cross, but could only head straight at Oblak.
Benfica - unbeaten since November - then caught us on the break with 93 minutes on the clock, the otherwise outstanding Sandro tripped Lima in the box and the same player slotted home a penalty for 2-2 on the night, 5-3 on aggregate.
So we went out of the competition - but with our heads held high.
"We knew after conceding the goal and going 1-0 down we had to score a minimum of three to take it into extra time," reflected Nacer.
"That was tough for us but we knew if we scored one, we could do it.
"After the first goal, we all started to believe. We pushed and pushed and after the second goal, I really felt we could do it.
"We had chances after that, Gylfi's header was a great save and we gave everything. It was such a shame they scored right at the end."
Nacer was delighted with his double, not least after his chances in Sunday's derby, but said he'd swap them for a place in the quarter-finals. "I'd prefer not to score and for us to have gone through," he said. "It's all about the team.
"All we can do now is move onto the next game, Southampton on Sunday. We have to go for it."