IF Ronny Deila is climbing with Celtic then he has reached a point of the ascent where a slip is likely.

At Parkhead tonight his side will expose its excellent recent results to an opponent capable of applying a handbrake to their momentum. Only Legia Warsaw had greater weaponry to test Celtic than the Red Bull Salzburg team which will turn out in Glasgow this evening.

It was to Celtic's credit that they took a 2-2 draw when the sides met in Austria in September, because since then Red Bull have shown themselves to be the strongest of the four teams in Europa League's Group D. Craig Gordon's saves spared Celtic from a defeat in that opening game.

A strong team has a strong goalkeeper - he isn't there by some stroke of good fortune - but the fact that he was so busy was proof of how difficult Deila's men found that game. Red Bull are the Austrian champions, they are six points clear in their league, and it's only nine months since they beat Ajax 6-1 on aggregate in last season's Europa League.

Deila spoke about passion yesterday when he was asked about his exuberant celebrations after Celtic won thanks to a late goal at Aberdeen earlier this month. There were echoes of that infamous strip show from his days at Stromsgodset. He was more reserved at Pittodrie - perhaps it was colder - but there was still an obvious letting loose after the relief of passing a major test. If that was how he reacted after his team went top of the SPFL Premiership for the first time, presumably he won't be shy and retiring if Celtic get the result which qualifies them for the round of 32 tonight.

If they beat Red Bull, or Dinamo Zagreb fail to beat Astra Giurgiu in Romania, they will be playing European football after Christmas and Deila will have delivered his first significant accomplishment as manager. "Passion is important but you have to think right," he said.

"If you have too much emotion, you can't think properly. But after the game at Aberdeen it was time to let emotions out. I'm an emotional guy and I want to connect with the fans.

"It's a part of me I've shown before and would be nothing new for the people of Norway. So far there has been a lot of assessing but when things are clear it's easier to get emotional, and also when you get wins like that. I want more moments like that in Aberdeen."

Since that victory in the north-east Delia has felt a different attitude from the Celtic support, perhaps a greater sense of acceptance. "Days like that bring us closer together. You have to earn your respect and I have to do the same as well. When you come from Norway into Britain you have to work hard. It's different from coming from Borussia Dortmund or somewhere.

"Now I have to show that I can do it at this level. You have to learn the culture. I have to adapt to the culture here as well and vice versa. That has taken some time but I understand things here more and more."

Legia took Celtic apart over two legs and Red Bull have the power to inflict further harm tonight. Delia spoke with undisguised respect for them, predicting an intense game with high energy levels and Red Bull pressing the home side aggressively and relentlessly.

Celtic don't expect to have a minute's peace when they have the ball and the Austrians will try to smother them when they play it from the back. Celtic's ability to cope with that, and to minimise their own errors and assert some authority in the game, will determine whether they are spared their third defeat in six European home games this season. "You could say they are Champions League level," said Deila. "Their level is quite equal with Maribor and Legia Warsaw [who won at Parkhead and Murrayfield respectively]. Maybe, along with Legia, they are the best team we have faced so far in my time as manager. We need to step up because it is going to be a tough test."

Celtic's season has reached a little crossroads, though. If results go their way tonight they will have qualified with a group game to spare, and if they defeat Hearts on Sunday they will march on to the next round of the Scottish Cup. They would enter 2015 fighting on four fronts. Being able to sell themselves as a club still in the European knock-out stages would be particularly beneficial when they are trying to lure players during the January transfer window.

"It's positive to go through and makes it easier to get players. We want to build a team which is progressing. We are going to add and it has to be the right players. It could be four, it could be two. If we take loans, we must have option to buy so they can become our players."

Players who might become his can wait. Today he faces a late decision on whether Jason Denayer can play after taking a knock against Dundee. Wakaso Mubarak is unlikely to play because of illness and Mikael Lustig is out. Red Bull score goals and missing one or possible two of their strongest back four is a problem against a prolific forward like Jonathan Soriano.

Celtic could do with there being truth in the rumours that Red Bull - who have already qualified but have not yet the group - plan to field a weakened team. Whoever is selected, Deila's men face one of the most demanding nights of their season.