The Belgian said they lacked confidence to attack the LaLiga Champions.
The former Everton star said that United will find it more difficult to stage a comeback against Barcelona than the one they managed against Paris Saint-Germain.
Man United booked their place in the Champions League quarter-finals with a remarkable 3-1 victory over PSG last month and were rewarded with a tricky tie against Barcelona.
But Ole Gunnar Solskjaer’s side face another uphill battle after Lionel Messi-led Barca left Old Trafford on Wednesday evening with a 1-0 win.
Lukaku has stressed that his side need to show significant improvement in the return leg at Camp Nou next week.
“Barca are not PSG, they have more experience,” Lukaku told COPE.
“We have to be confident and go to the Camp Nou with passion and generate more chances.
“We lacked confidence with the ball, I hope that next week we will play better than we did today.”
