Mexican Perez, who has switched from his 2012 employer Sauber, told Sport Bild he thinks Mercedes' Rosberg - the winner of one grand prix - is ultimately quicker than reigning three-time world champion Sebastian Vettel.
"In terms of pure speed, Nico Rosberg is the fastest German," Perez is quoted as saying.
"But Sebastian is the more complete driver and a great champion," the 22-year-old added.
Perez told Sport Bild he intends to push to beat Vettel and all other adversaries for the 2013 drivers' title.
"I am 100 per cent sure that the McLaren will be better than my Sauber," he said, "which is why I need to set high goals.
"They (McLaren) have the money, they have the people."
And Perez says he is already better than he was early last season, when only a driver mistake stopped him winning in Malaysia.
"If I could do that race again, I would win it," he insisted.