The Finn has struggled notably in recent races, and has spoken openly to the media about his dip in form.
Rosberg, attending the Japanese grand prix as a television pundit, is quoted by Osterreich newspaper as saying Bottas is in a "small crisis".
"From my point of view, it is not ideal that Valtteri talks publicly about his problems, because that creates a wave of its own and it doesn't help.
"I think it's better to keep that to yourself but he's going to do it in his own way," Rosberg added. "He's a great driver."
Bottas rejected any criticism of his approach.
"Normally I just say things how they are," he said. "I'm not really interested in what people say.
"I just know my own targets, my own goals and if I'm far away from those I'm not happy and then it's tricky."
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