Henrik Larsson insisted that he should have been given two goals as Celtic were held to a 1-1 draw by Villarreal.
The Swedish striker headed his team's equaliser but had one effort ruled out for handball when he charged down a clearance from keeper Jose Reina.
"It was definitely not a handball," Larsson told BBC Sport, although television replays suggest otherwise.
"It was a very frustrating night but we never gave up and we got a goal back - two goals actually - but that's life."
The Spanish side went in front after nine minutes thanks to a headed goal from Josico and the visitors played some slick football in the first 45 minutes.
"We let them in with an early goal but it is still an open tie," Larsson added.
"We would have liked to have kept a clean-sheet but that is not the case and we have to deal with it.
"It's always difficult when you concede a goal but we will give it our best shot over there.
"And let's hope we get a little bit of luck in the second game."
Celtic will have to do without Alan Thompson for the return leg after the midfielder picked up a yellow card late in the game.
But manager Martin O'Neill shrugged off the loss of the England international.
"We have had to do without a lot of key players in a lot of key games in Europe in recent seasons," O'Neill told BBC Sport.
"It's hard work but there is nothing we can do about it. Someone else will come in and fill the breach."
O'Neill was concerned that the referee had missed a foul on Stilian Petrov in the build-up to the Villarreal goal.
"It looked as if Stan had been fouled beforehand but, because he tried to stay on his feet, the referee waved play on," O'Neill continued.
"Eventually, they got up the field and forced a corner to score from that.
"But you can talk about individual decisions until you are blue in the face and, overall, I'm not overly bothered about that.
"In the second half, we were terrific. We played some great football on a brilliant surface.
"It must have been a great game to watch and we fought our way back into it.
"Villarreal are a very good side and they have proved that. It will be difficult but we will go there with plenty of confidence."