Martin O’Neill was asked the question many Celtic supporters have been debating for years after the 4-1 defeat to Benfica.

Why have Celtic gone from a club capable of winning regularly in the Champions League to one now finding Europa League games so difficult?

It is not a question with a quick answer, and O’Neill did not pretend otherwise.

The Benfica result sharpened the debate again.

Celtic were 3-0 down at half-time, and although Camilo Durán pulled one back after the break, Vangelis Pavlidis added a late fourth for the Portuguese side.

After the match, O’Neill was asked directly who or what he thinks is to blame for Celtic’s decline in European fortunes over the years.

Speaking to BBC Sport, he said: “That’s a big, big question. I haven’t seen all the things that have happened over the last 20 odd years.

“I have just spoken to the Benfica manager before the game and naturally, although he wasn’t here at the time, but they are massively disappointed that they’re not in the Champions League and they could give a good account of themselves.

“There’s some really serious teams in this Europa League and, that aside, your question demands a longer answer than I can give at this minute.”

It was a careful answer, but still an interesting one.

O’Neill did not dismiss the question. He also did not offer an easy excuse.

By bringing up Benfica’s disappointment at being in the Europa League, he pointed to the level Celtic are now facing even in Europe’s second competition.

This is no soft landing.

Benfica had the quality to punish Celtic ruthlessly in the first half, and that is the reality O’Neill’s side are now dealing with.

Still, supporters will look at the wider picture.

Celtic’s European record has been a sore point for too long, and the frustration is not only about one night in Lisbon.

The club have had memorable results since the Seville era, but not enough sustained progress.

Too often, Celtic have had good spells in games without turning them into results.

That happened again against Benfica.

Celtic have six games left to save this campaign.

O’Neill may not have had time to give a full answer on years of European decline, but the question will not disappear until Celtic start winning these matches again.