Chelsea meet United at Old Trafford on Saturday, having failed to win at the stadium in 12 years.
Maresca began this morning's media conference stating: "I think it's always difficult to play United away at their stadium. Now is not the right moment, next year or last year, it is always difficult.
"Some of the games they have played this season is not the real result, even the City game. It will be a tough game, no doubt."
On facing a struggling United, Maresca was wary.
He continued: "They have the desire to win this game after when you struggle, there is a desire to win the next game. Against Chelsea, at home, in front of their fans - they are all reasons why this game is important."
Bayern result
Chelsea go to Old Trafford on the back of their Champions League defeat at Bayern Munich. However, Maresca drew plenty of positives from their performance in Munich.
He said, "I think Wednesday's game is a normal game in terms of a Champions League game, Bayern away, we knew that was a tough game.
"We tried our best. For part of the game, we were very good and then for sure, there are things we can do better like always. My feeling at the end of the game was a good feeling in terms of we are here and we will play our part in the competition."
Garnacho to start?
Meanwhile, Maresca admits he plans to throw Alejando Garnacho into the lion's den. The former United winger, says his manager, is now ready to start tomorrow night.
Maresca said, "In the last two games, he was very good when he came on. So we are happy with the way he has adapted to our style and what we want from the winger. I think he is ready to start."
On fellow new arrival Estevao Willian, Maresca was keen to urge patience, though also stated the teenage Brazilian is already producing "great" moments.
He continued: "Estevao can be great in all of the attacking positions. His talent is very good, he is a top player, but he is 18. He needs time."
Maresca also confirmed Cole Palmer was fit to start after his return to action in Munich.
"Cole is okay. We have a session now, we flew back yesterday. We need to assess some players, we have some doubt. We didn't have a session since Wednesday's game, but we will see."
Sterling and Disasi
Before signing off, Maresca addressed the controversy over frozen out pair Raheem Sterling and Axel Disasi. The PFA have expressed concern over the way Chelsea have treated both senior players since the start of preseason over the summer.
Maresca, however, defended his and the club's actions, stating: "I've been in Raheem's situation and Axel's situation as a player. I know it is not the best feeling. You want to play games but for different reasons, the situation is the way it is at the moment. The club is giving them the opportunity to work in the right way. That's all I can say.
"I know it is something you like and want to talk about. It's not just Chelsea, it's every club in the world. When for any reason, the player and the club does not find any solution; you give the player the tools to train but if you are not involved in the squad, you are not involved in the squad."