Juan Mata scored twice and Javier Hernandez and Adnan Januzaj also found the target as the Red Devils ran out emphatic winners.
The result extended Newcastle's losing run to three games, having also been beaten 3-0 by Everton and 4-0 at Southampton, and left Pardew frustrated.
"It was difficult because we were coming off the back of a couple of bad defeats," Pardew told Sky Sports.
"The first goal was important today and their tails went up once they got it.
"We made a mistake for the second goal and then we were really chasing it and we had a problem.
"It is a problem for us, scoring goals. We've had a couple of big chances today and I think we just need a goal to give us a lift.
"United were good, you have to say, they settled down and played well.
"We are lacking a bit of positivity on the pitch and we need to try and get that in training this week."
Pardew is also hoping to be boosted by the return to fitness of Loic Remy, insisting that it would be unfair to suggest Newcastle have had a disappointing campaign.
He said: "We are getting tagged with having a poor season but we have had a great season until we lost Loic to his last injury.
"It would be nice to get him back and I think he is close for Stoke."