The Scottish champions were beaten by Shakhtar Donetsk in their group opener in Ukraine on Tuesday night, losing two goals in the opening eight minutes.

The experience would have rattled the new players in particular, who have now had their first taste of defeat after celebrating moving top of the Clydesdale Bank Premier League last Saturday.

But Celtic coach Burns warned they have no option but to take the bad times on the chin and regroup quickly before resuming league action at Easter Road on Sunday.

"In football, it isn't always sunshine," he said. "Sometimes the rain comes and that's what happened the other night.

"If they want to be a footballer, and play at a high level with big demands - which is what is expected of them here - then they have to develop as men also.

"They have to be strong and go out and try to get the three points on Sunday and make it up to the supporters and get the confidence back up again."

Celtic reached the last 16 of the Champions League last season, while Rangers opened their own campaign in style with a 2-1 win over Stuttgart at Ibrox on Wednesday night.

However, Burns feels the occasional reality check is needed to remind Scottish football fans that their teams are being asked to operate on an entirely different level when they take to the European stage.

"Here, we have the ball for 90% of the time and the players are used to that," he pointed out.

"Then you go to that level and sometimes the opposition has the ball more than you do and it's a different mentality, you need real concentration when you don't have the ball.

"It's alien to the way we play here, it's a different world. It's one hopefully the players can experience and benefit from.

"The Old Firm operate on a different level entirely here because they are the clubs who can pay big salaries and spend big money compared to the other teams.

"It's very uneven for teams like Gretna to play Celtic and Rangers and expect to win, and equally it's the same for us against teams like AC Milan and Barcelona.

"That's not to say that the Old Firm can't have good results - we had good results last year and Rangers had a fantastic result last night.

"But, on a regular basis, it's much more difficult."

The huge expectations on managers to achieve success has been highlighted by Jose Mourinho's departure from Chelsea, just 24 hours after the Blues drew their own Champions League opener with Rosenborg at Stamford Bridge.

"It's the nature of football," shrugged Burns. "There are big expectations and you get well paid for that.

"Football management is a lucrative job but there is a price to pay.

"I don't think anything surprises me in football now. The undercurrent that seems to have been there for the last year or so was always going to come to the surface.

"I don't know the ins and outs of it but he has left now and it's great for the TV people, they will probably make a TV serial out of that.

"But he will move on to something else, someone else will come in and we will all be talking about something else next week."