Absent the invention of a time machine, we will never know the answer.

But, still, if there is anyone equipped with the knowledge as to where this current Celtic side fits into the pantheon of great teams, it is Martin O’Neill.

Debate was ignited this week by one his former players, Neil Lennon, who remains adamant that Ange Postecoglou’s all-conquering side would not be able to overcome that which was built by O’Neill. In terms of honours, the current Celtic still have a bit to go; a few more league titles, a treble and a European final should just about cover it.

Maybe he knew what was coming his way as he sat down at Hampden ahead of Sunday’s Scottish Cup semi-final against Rangers, but O’Neill’s verdict was distinctly diplomatic.

“Obviously I am biased because I thought we had a really, really good team,” he said. “Sometimes it is difficult to assess. But like everything else, if you are talking about my old favourite word, the benchmark, then the best side Celtic have ever had was the European Cup team of 67 obviously. Until any Celtic side wins the European Cup again those comparisons or contrasts are going to be a bit far-reaching aren’t they?

“But I thought we had a terrific team, really good side, very strong. And we had to be strong because I thought Rangers were really good. The Rangers side of the time I was there, you could not only have respect for them, you could fear them, really.”

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Not only were Postecoglou and O’Neill of different eras, but their teams were also contrasting in style. The unit which forged a path to 2003’s UEFA Cup final was a real powerhouse collective, imposing and physical all over the pitch.

Of course, they could play a bit, too, but Postecoglou’s Celtic are diminutive in comparison. O’Neill finds them ‘a joy to watch’, and is particularly impressed by how they approach each game with absence of fear.

The next step for them, he believes, is Europe. An unyielding adherence to their attacking principles left Celtic with a few bruises in this season’s Champions League, but O’Neill suspects they will be wiser for it come next term.

“It is a different side,” O’Neill said. “Essentially some of the midfield players and forwards are smaller but there is a lot of pace about the team and a lot of interaction between them as well. They have been a joy to watch, they have been really good. I think that they play the game with no fear.

“I think after a stodgy start to the manager’s time here he has pulled it around brilliantly and the team goes in now feeling it is capable of winning matches domestically, which they expect to do, but I think they can take that into European football next year.

“The Celtic side here, there’s a lot of pace in it which looks good.  

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“Some of the defenders are a decent size but midfield onwards you wouldn’t say are giants by any stretch of the imagination.  But they are finding ways. They play lovely intricate football. 

“They look as if they are enjoying it and why wouldn’t they because they are scoring goals?  It’s genuinely hard to draw some sort of comparison. I’d need to see them more often than I do. 

“I obviously tune in for the games – not just the Old Firm ones – and they have been a joy to watch.  With a couple of adjustments, I think they could make a decent impact in the Champions League which would be lovely.”

On the theme of comparisons, Kyogo Furuhashi’s prolific goalscoring exploits over the last two campaigns have elevated him into conversations which also include the name Henrik Larsson. It has, however, been less drawing about drawing direct equivalence between the two, and more debating whether Kyogo is the best Celtic forward since the iconic Swede.

The Japanese forward is on 29 for the season, and is also making a real habit of scoring against Rangers. His connection with an adoring fanbase is another factor, and there has rarely been such adulation for a player in the post-Larsson era.

“There are comparisons at this minute with Kyogo,” said O’Neill. “He’s done really, really well.  He’s an excellent footballer who scores goals.  

“It’s difficult at this minute on terms of where he is and what he’s done in the gametophyte compare him to Larsson.  Some comparisons are being made at this minute. 

“If he continues to do it at some stage you can do that. But Larsson was, one, a proven winner and won the Champions League with Barcelona.  

“Listen, Celtic at this minute have some really talented players and anybody could argue that some could get into our side and some could argue that some of my team could get into this present side. 

“But I think we’d a really good team and I think Rangers were really good. Here’s one. I think the Rangers team of my time would beat its Rangers side.”

Even most of an Ibrox persuasion would agree that is a less controversial statement than comparing O’Neill and Postecoglou’s Celtic teams. The Rangers faced down by the Northern Irishman following his arrival from Leicester City was a fearsome one containing the likes of Ronald de Boer, Giovanni van Bronckhorst and Michael Mols.

Rangers, and Scottish football at large, does not boast such star quality these days. O’Neill also believes the league, as a whole, was of a greater standard than today, albeit he does suspect our game is on an upward trajectory.

“I honestly really don’t know what happened, it’s a shame,” he said. “First of all, I like to think it’s on the up.

“In my time up here, I thought the other sides were stronger. If you went to Aberdeen, you had a really difficult game, and I know Rangers found that out just last week.

“There was Dundee with the group of Georgian boys, they could turn up and do you. Going to Tannadice was really difficult.

“I don’t think the rest of the league is as strong now as when we were involved. Now, what has happened in that time? Would it be finance?

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“I don’t know whether there was a phenomenal amount of finance around in our time but Sky were covering the games. I think there then was an ask for more money and Sky decided to drop it.

“We generated money perhaps by playing in Europe. I don’t know all the reasons for it. I think the national team coming around again should help, which is great.”

Martin O’Neill was promoting Viaplay’s live and exclusive coverage of Rangers v Celtic on Sunday. Viaplay is offering a special limited-time offer for Scottish football fans available until Sunday only. Visit viaplay.com for more information.