Rashford has been in red-hot form this season, scoring 25 goals in 39 matches in all competitions for Erik ten Hag's side.
"It's pretty much impossible to be happy about something positive at Manchester United when you are the Liverpool manager - but I am really happy for Rashford," the German manager told reporters on Friday.
"He had a very difficult last year where he was not performing on the level that he is able to perform... Now he is playing incredible. His speed, his technique - it is a mix of everything. How calm he is in front of goal.
"He scores worldies, he scores the simple ones, he can put his head in. All these kinds of things. We have to defend against that collectively. He is not their only world-class player."
Liverpool are sixth in the league on 39 points after 24 games, 10 points behind United, who are third.
The Merseyside club have struggled for consistency this season, with recent 2-0 league wins over Newcastle United and Wolverhampton Wanderers coming either side of a 5-2 home defeat to Real Madrid and a 0-0 draw at Crystal Palace.
United look a team reborn under Ten Hag, having won the League Cup last week to lift their first major trophy since 2017.
"They have turned into a results machine. They are squeezing results out with some really good performances. Top football, and if it is not going so well they still get results," Klopp said.
"That's why they are there. They are fully in a fight to win the league. That's not really important for Sunday because we are there as well when we try to be at our best.
"It's what I enjoy most (this type of game). Around these games we pull ourselves out of the sometimes not-so-nice reality, that is when football is the most important for those 95 or 98 minutes, I love that. The whole world will watch, it's a big game."
Klopp added that he will be able to call upon most of his squad for the game against United, except for Thiago Alcantara and Luis Diaz, who are both still sidelined following injuries.
