IT WAS comfortable, if a little tetchy, as Celtic saw off Rennes with a 2-0 victory at Parkhead.

However, the little altercations that took place across the field with Scott Brown and Nir Bitton – reactions to a feeling that the French side had been a little over-physical at times – perhaps offered an insight into the adrenalin and nervous tension that is bubbling under the surface at Celtic Park.

Wednesday night is what it is all about.

This low-key summer, with a pre-season spent at Lennoxtown and Paisley, was all about conserving energy, staying calm and being focused on the challenge that lies ahead.

As such, it is difficult to over-play the significance of what this week means to Celtic and their season.

Domestically, a Treble will remain the ambition – and one which Ronny Deila will happily talk up – but, ultimately, the goal for this campaign is Uefa Champions League involvement at the lucrative group stages.

Given that Celtic’s defence of their league title is effectively straightforward – they may meet challenges along the way, but few would expect anything other than a Parkhead title party come May – Champions League football is the environment where Deila and his side can prove just how good they are.

The Norwegian manager believes his side has improved significantly over the last 12 months, but the real traction in that statement lies in what they can do now when the pressure is on and the quality of opposition is far greater than what Celtic meet weekly.

Since Deila’s preparations for these qualifiers began at the turn of the year for Celtic, there may be some relief in actually meeting the games head-on now. But while the part-timers of Iceland, FC Stjarnan, were never expected to offer resistance to Celtic, Qarabag are a different proposition.

This is the game where the bar is raised considerably.

The Azerbaijan side are awkward opponents, and not only because of the near-on 7,000-mile round trip the Parkhead side face for the second leg a week on Wednesday.

Having acquitted themselves well in the Europa League last season, Qarabag are a side capable of asking significant questions of Deila’s Bhoys.

As such, the manager will wait with some trepidation for news of Charlie Mulgrew this week. The utility man has deputised at left-back in recent games for fear that Emilio Izaguirre’s defensive frailties prove costly in this environment.

“We hope Charlie will be fit, but we have to wait and see,” said Deila. “You always want to have everyone available to you, but we have a big squad and if we have to, we will deal with it.

“Stefan Johansen and Mikael Lustig, who missed out on against Rennes, will be fit for us, but Kris Commons will not be available. He has done some training, but it is still too early for him.”

While Saidy Janko played the second 45 minutes on Saturday and impressed at right-back – meaning that there looks to be an able deputy for the injury-ravaged Lustig – the left-back position remains something of an issue.

St Pauli's Marcel Halstenberg has been a consistent target this summer, but moves for the left-back have stalled. And Deila has admitted some frustration in his attempts to land further new arrivals this summer.

Asked if he was confident of adding to the squad prior to the closure of the transfer window, with a specific view to signing a left-back, the Celtic manager said: “We’ll see. We have Emilio and KT [Kieran Tierney]. It is very hard to get players who are better than what we have.

"We are still looking – we are always looking – but this is the squad we have and right now our minds are focused on Wednesday night.”

Realistically, progression to the group stages could be instrumental in realising further additions.

It is not just the significance of the £20million windfall which involvement in the group stages guarantees, but also the carrot of enticing players to the club with the prospect of involvement at the elite level of the Champions League.

Little wonder then that Deila’s main focus from Saturday’s game was in the composition of his defence.

“It was a good game for us to get,” he said. “We wanted to be good defensively and create chances. I think we were compact for 90 minutes, so it was a good run-out before the important one. It was good for me that we were solid looking defensively.

“That is so important. We need to be sharp against Qarabag. They are a very good side. They have some good players, they are well organised and they are very capable of causing us problems.

“We need the level to be high. The way we played against Inter Milan last season – that is the tempo we need. But we need to be better defensively.”

At the other end of the pitch, there are unresolved matters, too.

Leigh Griffiths has made it difficult for Deila to stick to Nadir Ciftci up front on his own. The Turk can be aggressive and combative in that role, but when he appeared against Rennes on Saturday in the second period, he sat deeper and deeper as the game went on.

If he is to score goals, he needs to get himself into the penalty area.

By contrast, Griffiths has underlined his credentials as the club’s natural goalscorer with five goals in his last three outings.

It ought to be sufficient to have played his way into the starting line-up given the necessity of Celtic pressing home a first-leg advantage.