The French side face United in the first leg of their Champions League meeting on Wednesday night.
“We do not think of Manchester United as leaders of the Premier League but as just another team for us to play face-to-face, without fear and with fight from the first moment,” the Argentinian midfielder told Sky Sports.
“For us to defeat United some risks are necessary and we need to be a valiant team.
“Manchester United are at a high level because of the names in their squad but they could fail and I hope that happens against Marseille.”
“We are not obsessed with Rooney,” Lucho added. “He is not their only striker and it would be a big error for us to think that.
“Marseille also have a good group but in the Champions League it is necessary to have a little luck too, especially against the big clubs.
“I want to experience the satisfaction of beating a team like Manchester United. During my time in Portugal I played in some big matches but we didn’t manage to get the better of the Red Devils. It is not easy but now this is my challenge.
“Our aim is not to concede a goal at the Velodrome and to fly to Old Trafford with a good chance.”