Gordon Strachan has told his team that playing against United in the Champions League is a step up from International football.
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.'
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.'