Sir Alex Ferguson's men were humiliated by their bitter neighbours in front of the Old Trafford faithful last Sunday.
But more importantly, they lost ground on the title race as City stretched their lead at the top of the Barclays Premier League to five points.
And Park insists they need to get their title bid back and running again - starting at Goodison Park this Saturday.
"We have to put it right on the pitch and we have to prove it. It's a very important game to us and we have to go there and fight to get the points," the South Korean told the club's official website.
"Victory is the aim but it won't be an easy ride on a ground where United dropped two late points last season.
"It's not an easy game at Goodison Park - it's always difficult there," Park said. "Just look at last season, when we were winning 3-1 and in injury-time they scored two goals."
The 30-year-old also highlighted former United striker Louis Saha as the potential dangerman.
The Frenchman scored his first two goals of the season this week, and will hope to find the net against his former employers.
Park added: "We know his abilities are good when he's fit.
"Now he's on form and he's on fire, so we have to be aware of that. But we can deal with it because we know him.
"As a team we have to fight, not just individually, so we can stop them as a team. Then we have to think about our attacking side as well as our defence."
Tom Cleverley and Nemanja Vidic are among those expected to press for a starting place after proving their fitness against the Shots, while Dimitar Berbatov and Michael Owen also put in good performances.
Ferguson admitted after Aldershot: "It's difficult picking the team as we have a fantastic squad with so many good players. They've not had many first-team chances because the consistency of the side has meant I haven't had to make many changes."