Celtic manager Gordon Strachan has warned his players that playing Manchester United in the Champions League is a step up in class even from international football.
The Scottish Premier League leaders begin their Group F campaign at Old Trafford on Wednesday.
Sir Alex Ferguson's men have started the season with four consecutive victories and Strachan admits his side face a stiff challenge against top opposition.
He said: 'We are going to play against some of the best players in the world.
'Forget about international level - that really doesn't count.
'If United play in the English Premiership and then play at international level then 90% of the time they are going down a level. This is the highest level you can play.'
Strachan admits his inexperienced defence will be under immense pressure.
Goalkeeper Artur Boruc, Mark Wilson, Gary Caldwell, Stephen McManus and Lee Naylor will be looking to keep Wayne Rooney, Ryan Giggs and co quiet.
Strachan said: 'If we look at our back four they are young in the Champions League.
'If you look at AC Milan a couple of years ago their average age was 36 and we have an average age of 25.
'It's hard for them but the good thing about it is they got a taste of what the surface and atmosphere are like when we played there at the end of last season (losing 1-0 in Roy Keane's testimonial in May). We will need to protect them.'
Strachan admits his side need to ride their luck this week if they are to pull off a shock in Manchester.
He added: 'We are playing against a team who have won all their games so far and are on fire at the moment.
'Their players are playing well. We know it's going to be hard but we have to believe that we have the players to give them problems.
'We are going to need a bit of luck but I have never known anybody to go to Old Trafford who haven't needed a bit of luck to get a result.'
But Strachan is more optimistic now than he was during the pre-season when they failed to win a single game.
Celtic have lifted spirits by signing Thomas Gravesen and Jan Vennegoor of Hesselink.
Strachan told Scottish TV: 'We are getting better as the season progresses.
'When we first set out in the pre-season and in Poland we had a bunch of young boys. Personalities had left the club and a lot were about to leave the club.
'I was a bit worried if that team would be playing but I am getting more confident as the season goes on.
'I am more confident after Saturday (1-0 victory over Aberdeen at Pittodrie) against a side who were unbeaten in the league and had not lost at home since December.'