Niki Lauda says he feels relaxed now that Valtteri Bottas has been signed up for 2017.
Earlier, the F1 legend and Mercedes team chairman sounded almost angry that Rosberg had left the world champions in the lurch by quitting after winning his title.
"My tension, which had resulted from the withdrawal of Nico, has completely disappeared," Lauda told the German broadcaster RTL.
"Bottas is the best man for the job."
Mercedes had briefly looked into trying to secure a top driver to pair with Lewis Hamilton, and also considered promoting its junior Pascal Wehrlein.
Lauda explained: "Experience and speed were the reasons we signed Bottas. I believe he can drive as fast as Nico. I believe he can win the world championship."