With the tie finely balanced following a 0-0 draw in France, Marseille head to Old Trafford on Tuesday knowing their English hosts have not lost there all season and are favourites to reach their fifth successive quarter-final.
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"If you look purely from a statistical point of view, Manchester United have not been beaten at home (since April 2010), they only conceded one goal in seven Champions League games this season, so it's a tough task," Deschamps told a news conference.
"On paper you would expect fully that Manchester United will qualify but we have our chance, our little opportunity. And football is not about statistics alone."
He said France striker Andre-Pierre Gignac, who struggled in Marseille's 2-0 league win over Stade Rennes on Friday after returning from injury, could be key to their plans, while United winger Nani's surprise return from injury could be damaging.
"Gignac started this year very very well and has had 10 days away from the game through injury. I think he is possibly going to have an important role tomorrow night," said Deschamps.
"Nani did well in the first leg, he caused us one or two problems in the Stade Velodrome," the Frenchman said of the United player who is poised to feature despite having been ruled out of contention last week because of a gashed leg.
"He speeds the game up, attacks are rapid when they involve Nani. You can replace Nani, with (Wayne) Rooney, Chicharito (Javier Hernandez), also (Ryan) Giggs... any number of attacking options that United have got which really form this attacking armada can create problems for us at Old Trafford."
NEVER LOST
Further statistics Marseille will want to forget are that United have never lost to French opponents at home and that a team who go into a home second leg having secured a 0-0 away draw will usually progress.
"It was a good result for Manchester United... if you look again at stats, if you get an away draw and do not concede, around 70 percent of the time the team goes through when they play at home," Deschamps said.
"We are talking about Manchester United here, they know how to manage whatever situation might occur on a European occasion," he said of the three-times continental champions before drawing comfort from the 0-0 scoreline.
"They know they are more obliged to go on the attack when they are at home and it's tough having not scored away if they do happen to concede at home it does complicate the issue a little bit for them."
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