Arsene Wenger's men are currently at the top of the Premier League and will clash with United at Old Trafford on Monday looking to strengthen their hold at the summit.
But a victory for the Red Devils will see them go to the top with a game in hand, as last week's game against Blackpool was postponed due to bad weather.
"Arsenal are a bigger threat than Chelsea now," Nani told reporters.
"Chelsea have lost a few games and drawn some and their confidence looks a bit low.
"But Arsenal have been winning and being top of the league gives you great confidence."
The Gunners have lost only one of their eight away games this term, winning five and drawing two. They have also scored 15 goals on their travels, which is the most that any team has scored away from home.
And the Portugal international believes that a part of it is down to Samir Nasri, who has been in roaring form during the course of this campaign.
"Arsenal have been doing well. They are growing up in the way we are," he said. "Nasri is the most important player in the Arsenal team," said Nani.
"He is a very big danger for us and we have to take care of him."
But for his own part, Nani is relishing the prospect of taking on the north London club on Monday.
He said: "I like to play against Arsenal.
"It is good playing against any big team but I have done well against them in the past.
"The way they play gives you a chance to show your quality."
United remain as the only team yet to be beaten in the Premier League, but seven of their 15 games have ended in draws.
However, Sir Alex Ferguson's charges' last league game was a 7-1 demolition of Blackburn Rovers.
"Our momentum is growing," Nani said. "Players like Wayne Rooney and Anderson are starting to play fantastically again.
"It is great because our next two games are going to be very important for us.
"If we win against Arsenal and Chelsea we will be in a very good position to win the league."
