Rosberg suddenly quit at the end of last season, plunging Mercedes into a desperate search to replace him.
The German admits that his decision might have hurt Mercedes.
"I wouldn't exactly say that," he told Auto Bild, "but of course my departure was difficult for them, especially with the new regulations."
Indeed, Mercedes started 2017 on the back foot, but Rosberg noted: "It's nothing against Ferrari, but they were able to start with their new car much earlier.
"So that was their advantage. But Mercedes is still very, very strong - when you look at the car, the level of detail is just amazing."
Rosberg also said that he has been impressed with Mercedes's choice to replace him - the former Williams driver and Toto Wolff protege, Bottas.
"I'm really impressed," he said. "He might be the perfect driver mentally because he is able to just concentrate on himself, making him consistent and fast."
As for the high-profile Hamilton vs. Vettel battle, Bottas said: "That is perfectly ok for me! I have no desire to be the centre of attention.
"It means I can just concentrate on my job."
The season emerges from its summer break on August 27 with the Belgian Grand Prix.