Nuno Espirito Santo was left to rue the "small margins" after watching his Wolves side bow out of the Europa League with a 1-0 defeat to Sevilla in Duisburg.
The Midlands outfit had a chance to take the lead when Adama Traore was brought down inside the box by Diego Carlos in the 12th minute.
Raul Jimenez, with a perfect penalty record, stepped up only to be denied after a poorly-struck effort was kept out by Yassine Bounou.
But replays showed that the Sevilla goalkeeper was off his line when the penalty was taken, while several players from Sevilla were guilty of encroachment before the spot-kick - but neither incident was picked up by VAR.
Lucas Ocampos won it with a header from Ever Banega's cross to set up a semi-final clash with Manchester United on Sunday and Nuno rued the "small margins" which cost his side.
He told BT Sport: "Frustration, in the last minute something that happen to us too many times. You require focus from the set-piece, but I'm proud of the boys.
"We played a tough team, we had moments and it is all about small margins. We came this far, now it is over so let's rest and look to the future.
"It is a small squad, we must be very proud and now we need to make good decisions, so we cannot make mistakes like we did before and we need more players that can help us. For sure the future is bright for us."
Asked about the penalty incident, Nuno added: "I didn't see the images but if you tell me that, VAR shouldn't have allowed it. I didn't see the image so I can't give an opinion on it.
"We cannot do anything about it. It is one detail that will make us even more sad than we are now.
