David Moyes has been under increasing pressure with United suffering two straight losses to slip to ninth in the English Premier League.
But Moyes' men top Group A in the UEFA Champions League and Lucescu warned his team United were a different prospect in European competition.
"Even though they are not playing well, these are two different competitions and you can't really compare them," he said.
"In the Premier League, the English teams know each other and the teams they are playing, but with the Champions League it is a totally different situation.
"For Manchester United it is a must-win match as they want to win so they can top the group and for our part we are going to play our best as it is important for us to progress as well."
Lucescu expects Moyes to rotate his squad with an eye to Sunday's Premier League trip to Aston Villa, but said even then United were a strong side.
"Every manager knows their squad back to front and each individual player, and each manager has his own style," he said.
"Some prefer to rotate to keep the squad fresh and some prefer to keep the same team.
"In David Moyes' case he likes to rotate his team but the squad is very strong and therefore the quality of the players doesn't diminish."
United suffered a 1-0 loss at home to Everton on Wednesday before slumping to a 1-0 defeat against Newcastle at Old Trafford on Saturday.
Lucescu noticed United had 'lost their spark' against Newcastle, but believes they were still close to taking something out of the clash.
"When they were playing Everton it was David Moyes' former team so they were playing with special enthusiasm and motivation, but when they played Newcastle it was like they had lost their spark a bit," he said.
"The result of the game does depend on some small things, though. For example, if they got a penalty for the handball against Newcastle, it could have been completely different."
