WHEN even the eternally-ebullient Kieran Tierney is staring crestfallen at his feet then you can safely deduce it has been a rare off-day for Celtic. The full-back has suffered precious few bumps in an otherwise smooth and steep career trajectory that has seen him moved through the ranks from the youth academy to the first team before becoming a fully-fledged international player.

For once, though, even Tierney was struggling to cling to the positives as he picked over the bones of Celtic’s opening Champions League defeat at home to Paris Saint-Germain. If it seemed harsh to point too many fingers at the Celtic defence following the 5-0 loss – PSG’s array of attacking talent will dance rings around far better sides this season – then it was not the stance adopted by manager Brendan Rodgers or his players who felt they should have made the Parisians work much harder for their win.

Celtic demonstrated their resolve last season when they bounced back from an even heavier loss to Barcelona in their first group game and the hope must be they will similarly improve this time around.

“It was a learning experience for us and we'll need to be better next time,” said Tierney. “They [Neymar, Kylian Mbappe and Edinson Cavani] were probably the best front three in the world. The money they spend is crazy. But the first half wasn’t good enough from us, we should have done better.

“Then in the second half we got better. We had half-an-hour when we were in it, then we tired and they got two quick goals which kind of ruined it. Tuesday night was a tough ask, but we know we need to get better.

“It’s hard to come and play against arguably the best and definitely the most expensive team in the world, even more so when it’s your first group game and you haven’t had time to settle in. You don’t get those chances at this level, you need to do it from the word go because once they score it’s always going to be hard to get yourself back in.”

At just 20, Tierney is still learning his trade. And up against the world’s most-expensive teenager in Mbappe he was certainly given a lesson.

“He’s so quick, you think you know what he’s going to do, but you just can’t get to him,” the full-back said. “I suppose that’s why they paid so much money for someone so young.

“As a team they don’t waste a ball. If it’s a high ball, we’d maybe look to get it clear, but they’ll bring it down on the chest and play it.

“You look at Dani Alves. There’ll be a ball over the top and he’s close to me and he’s comfortable about taking on the chest and passing it. If we tried to press them, their one and two-touch play was unbelievable. You think you’re going to get there and nick it, but they’re just touching it away and they’re on the break again. They’re a world-class team, no doubt about it. But we need to learn from it all and hopefully we will.”

Celtic are next in action against Anderlecht and Tierney knows they must be better prepared for that challenge.

“We need to pick ourselves up fast because there are games to face in Scotland as well as in Europe,” he added. “We can’t afford to be down for too long.

“We had a poor start last year in Barcelona, but we bounced back with a good result against Man City and we need to aim for the same again, 100 per cent. The boys know that. Anderlecht are a great team too - there are no easy games and especially when you’re in Pot Four you know every one, home and away, is going to be tough."

Rodgers hoped both Tierney and Tony Ralston, making his Champions League debut, will benefit from having experienced such a difficult night.

“For the two young full-backs….can you imagine the experience Tony Ralston got out of that and how well he has done which hopefully bodes well for him, Celtic and Scotland,” said the manager.

“Kieran as well. They were up against the two most expensive footballers in the world and they actually did really well so there will be success for us in different ways in this competition.”