United welcome the Ligue 1 leaders to Old Trafford for the first leg of the last 16 clash on February 12.
The Premier League side are in good form again after recovering from a difficult start to the season, winning nine of their 10 matches in all competitions since caretaker manager Ole Gunnar Solskjaer stepped in to replace the sacked Jose Mourinho in December.
The United boss was in the stands to see PSG lose in the league for the first time this season against Lyon on Sunday, and centre-back Thiago Silva said he expects the Norwegian to have come away from the game with a boost in confidence.
But Lukaku believes it is PSG who will be in better spirits heading into the encounter and would not be surprised if they went far into the competition.
"For me, they are the favourites, but we will come well prepared and we will see what will happen on the field," he told Canal+.
"We know we will play against a team that is, for me, one of the favourites in this tournament.
"You just have to see their squad, there are players with great talent and a good coach, good fans and leaders who have a great plan for their club. It will be a difficult match."
United's resurgence under Solskjaer has brought them back into contention for a top four finish in the Premier League, as they moved into fifth in the table with Sunday's 1-0 win over Leicester City to leave them two points behind fourth-placed Chelsea.
Lukaku has noticed a big change in spirits in the dressing room, saying star players like himself, Paul Pogba and Anthony Martial are happy again.
"Since the departure of Mourinho, it's different, it's much more positive," he said. "Already in the game people smile a lot more, right now everything is going very well, and we must continue like that. We are benefitting.
"[Pogba] is happy. In training, he is told all the time: 'Paul, we see you happy!' And after he laughs. So now, everything is going very well, everyone is playing at their level.
"Paul, Marcus, myself, Anthony, everyone, the competitive spirit is coming out in every player and it's already starting in training when we play small matches, six against six, eight against eight, it shows that the players want to prove themselves in front of the coach, because they want to seize their opportunity.
"When we play a game, we know that each player can play. But the most important thing is the team."
Before their meeting with Thomas Tuchel's side, the Red Devils travel to Fulham in the Premier League on Saturday as they aim to continue their fine recent run.