The visitors to Portugal endured a nightmare start as they conceded a penalty inside the opening few minutes before finding themselves 3-0 down during a damaging first half.

Celtic produced a much-improved display after the interval, however, creating a number of opportunities before Benfica eventually added a fourth on the break.

O'Neill believes that second-half showing demonstrated why Celtic should not give up hope of progressing with six games still remaining. He said: "I thought the second-half performance was really terrific, as you say, a game of two halves.

"We created chance after chance; we should have scored more goals, and of course you're going to leave yourself a bit open at the end.

"But I tell you, the second half performance gives me what I would call the determination, I think, to, and I think we can still qualify. I know it's down to six games, but we want to qualify for this competition to get into the knockout stages.

"We can still do it."

O'Neill was frustrated with the manner in which Celtic conceded during the opening period, particularly after seeing his side pass up an opportunity to reduce the deficit when trailing 2-0.

He added: "One is just a little dinking cross and we're coming out instead of defending it properly, and that's really poor from our viewpoint. You can't afford to do that.

"I don't think you can afford to do that in any game, never mind a game of this magnitude. But really, trying to pick up from that there, the second half, as I said to you, we could have capitulated, but it could have been all over.

"We came roaring back, the substitutions came on and did brilliantly, the substitutes did brilliantly, and we could have scored a number of goals."

O'Neill was particularly encouraged by the impact of Haissem Hassan and Callum Osmand from the bench, with both wide players willing to run directly at the Benfica defence.

He said: "It changed because two wide players wanted to get at people, really did want to get at people. Hassan has that ability to do this here, and little Osmand, who's just back from injury, hasn't played really wide there at all.

"I thought he did terrifically well. Maybe he could have squared one for Duran to maybe make it 3-2 in the game. That would have been really interesting in the last 15 minutes of the match.

"But even so, great determination to go beyond and also picking up and taking players on. And so much so that they replaced the full-back on the Benfica side because he was causing problems.

"So that gave the rest of the team a bit of impetus."

Despite the heavy defeat, O'Neill was adamant the positive aspects of Celtic's second-half display could be carried into their next fixture.

Asked whether he would take encouragement from it, he replied: "Absolutely, we'll take that.

"Yes, second-half performance I will definitely take."