A 3-1 win over Everton on Saturday set the league leaders up to ensure they cannot be caught by any of the chasing pack if they get the better of second-place United at the Etihad Stadium.
The comfortable advantage Pep Guardiola's side have held for most of the season has made the title seem a mere formality, but Laporte says they must go all out to make it a special day by inflicting double damage on their nearest challengers.
"Obviously that is a very important game on Saturday," the City centre-back said.
"I think it is a once-in-a-lifetime opportunity and we must take advantage of it. We must try to win and get the Premier League we all want."
The 23-year-old was used at left-back as City defeated Sam Allardyce's men at Goodison Park and could fill the role again when his side take on Liverpool in the first leg of their Champions League quarter-final tie on Wednesday.
Although Laporte felt comfortable on Saturday, he expects a difficult night at Anfield in midweek.
"It felt really good. It is a different position but we worked over the week in training sessions and I felt fine," he added.
"We fight every game like it is the last one and it was very important to win. You always try to play the best you can. But we need to keep on improving.
"We know Liverpool is a difficult game. They have great players and are in great shape. We need to be 100 percent focused to have a chance to win.'