The visitors to Portugal had a dismal start, conceding a penalty within the first few minutes and trailing 3-0 by half-time.

However, Celtic showed more promise after the interval, creating several chances before Benfica scored their fourth on the counter-attack.

O'Neill believes this second-half performance indicates Celtic's potential to progress with six games 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.

"We can still do it."

O'Neill was frustrated with the manner of Celtic's early concessions, especially after a missed opportunity to reduce the deficit at 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."

O'Neill was particularly pleased with the contributions of substitutes Haissem Hassan and Callum Osmand.

Both players were direct in their approach, causing problems for 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.

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

Despite the heavy defeat, O'Neill remains optimistic that the positive aspects of Celtic's second-half display can 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."