IT is open to debate, but for many Neil Lennon’s greatest achievements as Celtic manager were beating Barcelona and then qualifying for the knockout stages of the Champions League.

The Irishman won the Scottish title three seasons running and the Scottish Cup twice during his four whirlwind years in the dugout at Parkhead.

But the 2-1 triumph over Barca, a win sealed by a breakaway goal by 18-year-old substitute Tony Watt, in 2012 will never be forgotten by any Hoops fan.

And progressing to the last-16 of Europe’s elite club tournament for just the third time in Celtic’s history, too, was significant for the Glasgow giants.

Despite the disappointing defeat to Italian rivals Juventus that followed, simply making it to that point was a major accomplishment given the size of their budget.

Yet, it is often forgotten that Lennon had struggled badly in continental competition after succeeding Tony Mowbray and becoming a boss for the first time in 2010.

His side lost heavily to Braga of Portugal and Utrecht of the Netherlands in qualifiers for the Champions League and Europa League respectively in his first full season.

The following campaign wasn’t much better either - Celtic slumped to a 3-1 defeat to Sion of Switzerland in the Europa League play-off.

The Bhoys were only reinstated after Uefa discovered their opponents had fielded five ineligible players and awarded them two 3-0 wins.

However, draws against Udinese of Italy at home and away and a win and a draw over Rennes at home and away followed in the group stages.

After those outings, Lennon went on to enjoy some memorable moments against Helsingborgs, Shakhter Karagandy, Spartak Moscow, Ajax and, of course, Barcelona.

There are uncanny parallels between Lennon’s experiences in Europe as Celtic manager and those of his successor Ronny Deila.

Tomorrow night the Norwegian will embark on his second European campaign as Celtic manager when his side take on Icelandic minnows FC Stjarnan in the first of two qualifying rounds before, hopefully, they have to navigate a play-off hurdle.

But Deila has admitted the ruins of last year's European adventure still appear in his rear view mirrow, the scars still raw to the touch.

He looked through the cracks in his fingers as he saw his team thrashed by Legia Warsaw in the third Champions League qualifying round last year in just his third and fourth games in charge.

And then, after the Poles had been punished by Uefa for fielding an ineligible player, they lost to NK Maribor in the play-off game.

But they enjoyed some fine performances and results in the Europa League and made it through to the last 32 where they performed well against Italian giants Inter Milan.

The secondary competition is not where Celtic aspire to be playing – but it has provided Lennon and Deila and their players with invaluable experience.

Certainly, Adam Matthews, the former Celtic right-back who played extensively in both competitions in his four years in Scotland, believes it will prove beneficial in the new term.

The Welshman, who swapped Celtic for Sunderland last month, will be looking on from afar and hoping his old club can go the distance and reach the group stage this season.

And he believes the entire squad, and new players like Stuart Armstrong and Gary Mackay-Steven in particular, will be well-placed entering the double-header against Stjarnan.

“Playing in the last 32 of the Europa League last season was big for the boys,” he said. “The second leg especially was special. The San Siro is a massive stadium and Inter Milan are a great club.

“It was frustrating because the sending off (Virgil van Dijk was red carded for two bookable offences in the first half) spoiled the game.

“I thought Gary and Stuart dealt with both matches brilliantly to be honest. You would have thought they were European veterans with dozens of games at that level behind them.

“In the home game they reacted to the crowd really well and coped with the atmosphere they generated really well.

“They have done brilliantly since they have joined Celtic and playing against Inter Milan in the San Siro gave them invaluable experience of playing at a higher level in Europe.”

Mackay-Steven had a chance to score early on in second leg against Inter and was disappointed when he shot straight at keeper Juan Pablo Carrizo.

Passing up that opportunity proved costly for the visitors as Fredy Guarin netted a late winner for the Serie A club to send them through.

Matthews feels witnessing how fine the margins for error, and how refereeing decisions can often go against the visiting side away from home, will help Celtic in Europe.

“When we had 11 men I thought we had the better of the game against Inter,” he said. “Stuart and Gary were causing them problems with their pace.

“Gary had a good chance and brought out a good save from the keeper. I thought we were always in the game. If we had taken advantage of the pressure we enjoyed there may have been a different outcome.

“The red card changed things. Sometimes you get refereeing decisions going for you and sometimes you don’t. But in Europe you are lucky when decisions go for you.”

Former Hearts and Sunderland goalkeeper Craig Gordon was a huge player for Celtic in Europe last season despite having two years out of action due to injury.

Matthews is confident the Scotland internationalist can maintain his high standards when the new season kicks off with the game against Stjarnan tomorrow night.

“Craig was brilliant for us, not just in Europe, but in the league as well,” he said. “His performances were exceptional.

“The way he bounced back from his mistake in the first leg against Inter showed the character of the man. Hopefully he can keep performing like that for us in Europe this season.”

Matthews added: “We put the Inter defeat out of our system really quickly and went on to win the League Cup and the Premiership title.

“But I think we did really well. When we had 11 men we were on top. We defended far better than we did in the first leg, which was a big objective, and created a few chances.”