Liverpool, who are two points behind fourth-placed United, are unbeaten in 13 Premier League games after a 1-0 victory at Swansea City on Monday night.
There are still another eight games after and there is going to be twists and turns constantly I’m sure
The Reds can hoist themselves above the Red Devils and into the final Champions League spot with a precious victory on Merseyside this weekend.
Lallana, who moved to Liverpool in a £25m deal from Southampton last summer, urged the Reds to seize the upper hand in the race for a top-four finish.
Lallana told talkSPORT: “Psychologically for us it is massive.
“If we win we go above United and we get one foot into the top four.”
Lallana added: “There are still another eight games after and there is going to be twists and turns constantly I’m sure.
“Tottenham and Southampton are still in and around the fight as well so they are going to be wanting to pick up points to make it three teams out of six trying to get in the top four.
“It is exciting for the neutral and I’m sure it will go down to the wire.”
Liverpool have beaten Manchester City and Tottenham Hotspur during their impressive 13-game unbeaten run in the English top-flight.
The Reds are looking to qualify for the Champions League in successive season for the first time since 2009.
United, meanwhile, warmed up for their trip to Merseyside with an emphatic 3-0 win against Tottenham at Old Trafford on Sunday.
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