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This is where things start to look increasingly difficult for Celtic.
Manchester United are among the favourites to win the competition. They have quality players throughout the side and they have Rooney and van Persie up front—one of the most lethal strikeforces in Europe.
Are there any positives for Celtic, then?
In short, yes.
Man United failed to make it past the group stages last season, something which was practically unheard of. Although they managed it this time around, they qualified from about as straightforward a group as they could have been given.
What's more, United's defence has been rather porous this season, often conceding several goals a game and regularly looking shaky.
Finally, Celtic tend to fare better on their travels when they only have to journey a few hundred miles over the border. They cope better against British sides who play a style they're more familiar with. And remember, it wasn't so long ago that they beat Man United at Parkhead.
Tough. But not impossible.
Celtic's chances of going through: 15%