Brendan Rodgers' men exited the UEFA Champions League on Tuesday, drawing 1-1 at home to Basel to finish third in Group B.
Lambert, 32, said the trip to Old Trafford came at the ideal time for his side, who are ninth in the Premier League.
"I'm not saying everything can click into gear in one game and then it will all be fine," he said.
"But, after the week we've had, it is the perfect match to get the team rallied and get us going again.
"If we can't get ourselves up for United, we can't get ourselves up for any game."
Lambert told his team-mates they needed to understand the criticism after their disappointing Champions League campaign.
"The flak is flying," he said.
"It's something that you can't get away from being a Liverpool player, something you have to accept.
"We were all expecting it after Tuesday and you have to put that on your shoulders and move on. Believe me, we are hurting just as much as the fans – and will try to rectify it.
"There is not much more I can say because the performance and the result against United is all that matters now."
With Daniel Sturridge and Mario Balotelli injured, and Fabio Borini out of favour, Lambert has started six straight matches in all competitions.
He is excited about featuring in a Liverpool-United clash and said there was hatred between the two clubs.
"It has turned into probably our most important game over the years. It's brilliant to watch, the atmosphere; the intensity of the game," Lambert said.
"There's a lot of respect but a bit of hatred as well. It's everything we want in a game. Two good teams, two top clubs going head to head. It's something I’m looking forward to."
