The German won last April's Chinese grand prix, but thereafter the Brackley based team suffered a collapse in car development and fell badly off the pace.
New teammate Lewis Hamilton has been playing down expectations following the new W04's launch, but Rosberg - who has driven for Mercedes alongside Michael Schumacher since 2010 - is more outwardly confident.
"I just want to win races," he told Finland's Turun Sanomat newspaper, "and I am very confident I can do it pretty quickly with this car.
"We still need to catch up," the German, whose father is the 1982 world champion and Finn Keke Rosberg, warned.
So, to Spain's Marca newspaper, Rosberg played down hopes Mercedes could fly out of the box in 2013 and immediately start charging for the championship.
But asked if the W04 can ultimately win multiple races this year, he said: "It's what we want to do, but at the end of last season we lost a second a lap. It's very difficult to make up that difference in one winter.
"You have to bear in mind that all the teams are improving their cars. But I think we're much closer and the speed at which we can develop the car now looks very good.
"That's what we were not good at last year."
Ultimately, Rosberg said the real pecking order will only be known in Australia, the scene of next month's 2013 opener.
He does, however, expect Red Bull to lead the field.
"I think so, more or less," said the 27-year-old. "For sure Red Bull is the team to beat."