You are what you eat, apparently.

Waddle up to the supermarket check-out these days and you half expect the food police to halt your advance and begin rummaging through the contents of your basket while tutting, grimacing and hissing with disgust at the collection of pies, cakes and microwaveable swill that form the bedrock of your messages.

Ronny Deila is not quite Gillian McKeith but the new Celtic manager has certainly given his players food for thought. "Things have changed here, the way we eat," defender Mikael Lustig, said. "To be fair it was Coke and Irn-Bru for lunch for some of the lads; we're having more food-wise now, thinking about how we can be better when we're not playing."

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Deila has his own way of doing things. It is a particularly meticulous approach. The Norwegian has his philosophies on how the game should be played and the standards he expects from his players, both on and off the pitch, are exceedingly high. Food, fitness, you name it. Deila is a demanding manager.

"We look at the body fat," he said, sounding like the supervisor of the Lennoxtown branch of WeightWatchers. "I want a quick team and you can't be quick if you weigh two or three kilos too much.

"You need quick players in Europe. Look at Ronaldo and [Gareth] Bale. That's the standard we're talking about. You talk to me about the Champions League? OK, then we have to look like a Champions League team and we have to train like a Champions League team.

Players who don't develop as players? I don't give a s**t if they make mistakes, tomorrow, next weekend or the weekend after that and we lose. But if you are the same player in the autumn as you were in the spring, then you have a problem and I have a problem"

The 6-1 aggregate qualifying defeat to Legia Warsaw was certainly not part of the grand plan, of course, and Deila's open, expansive approach and general tactical nous was severely criticised.

Celtic have a second bite at the cash-laden cherry, following the Poles' exclusion from the competition for fielding an ineligible player, and a visit to Slovenia to face Maribor in the play-off round now beckons on Wednesday.

The promised land of the group stage is looming seductively on the horizon and in order to get there, the uncompromising Deila knows he may have to, well, compromise some of his footballing ideals and beliefs.

"I think in Europe I have to because it's short-term and it's about results," said Deila, who has always preferred to take the long-term view and remains a great advocate of nurturing young talent. "If you go into the group stage then you have reached a bit of the goal.

"Then you can learn from the matches and at home you can be more offensive. Away you have to adapt more and be more careful. But these two games, of course, we have to put out the team that gives us the best opportunity of winning. It's not about development in those games, it's about winning and that's how I have to go into them I'm looking forward to the day that I can say to the players 'go and play normally'. That will be good."

Deila added: "I'm not here for money, I'm not here for trophies. I win a trophy and then don't look at it once afterwards, although of course it's what I remember. It's the small things. If I see a boy coming to the club and then he is a man leaving the club, I get goosebumps.

"I love to develop players, that what it is about for me. That's why I say I want to go to the stadium and entertain the fans. When I take my car home from Celtic Park and I'm so proud of the performance, and everybody's happy, then that's the ultimate for me.

"What's hard for me is that at big clubs, the pressure to get the result is so hard, so it's difficult to be creative because you're afraid of mistakes, If you make them, you lose, and then people are after you at once.

"That's why the best-driven clubs are the small clubs, because they can make mistakes, they can lose matches, but they can develop. That's why players come through. But when you come to a big club, everything is about winning."

Amid the tumult of Uefa's decision to give Legia the boot and usher Celtic back into the fray, the players are simply trying to focus on the job in hand. "It's a strange position to be in," Lustig admitted.

The whipping in Warsaw and the misery of Murrayfield in the second leg will be forgotten if Celtic can conjure success over the Slovenians. "We've been in the Champions League for two years in a row and we know how nice it is," Lustig added. "The fans love it, the club loves it and it's really important. It's a lot of pressure but we've been through [qualifying] for the last two years and we've been successful.

"We need to find our game now. We need to be better in defence as that's always been our strength during qualification. Both games against Legia we were too open. We should have played better but I can't explain why we didn't."

Deila will be hoping his players answer those questions and make sure the mouthwatering fare found at Europe's top table is on the menu for Celtic again this year.