From then on, with 25 minutes left, the Anfield men took their foot off the gas.
They had already overhauled United's once superior goal difference and have now scored more times than the champions this season.
"There were times after the fifth goal when we should have gone on and scored more," said Benitez.
"We should have kept going, scored more and made sure that our goal difference was better when we were playing against 10 men.
"You never know what we will need, how many more goals we could have scored during that period.
"Don't get me wrong. I was happy with the performance and the result, but we should have gone on and scored more."
The first came from the spot after a foul on Riera, and the second was a 20-yard free-kick after Kuyt had been felled. Gerrard then netted the fifth from the spot, after Friedel had been dismissed for bringing down Fernando Torres.
Benitez is content now that Liverpool have put themselves back into a position from which they can help shape their own destiny.
"As far as the title chase is concerned, we just have to keep doing our job - we must not be concerned about United's results and how they are playing," he believes.
"If United do make mistakes we must be ready, and that means carrying on doing what we are doing and winning points.
"We are playing well, and they will see that we are playing well and trying to win every game. We have eight games left, and every one must be won - and be treated as a cup final.
"Certainly we must maintain this momentum, and when we are playing well it just increases confidence.
"We now have an international break, so the players must come back from that, re-group and start playing again just as we are at the moment."