Although the reigning world champion, Hamilton looked out of form all weekend in China.
"It's a typical Lewis phase," Rosberg, who was paired with Hamilton at Mercedes between 2013 and 2016, told Globo.
"When things do not work out perfectly, he starts to lose his head and motivation a little and struggles for a while.
"You could see from his interviews that he was really suffering and didn't really understand what was happening.
"Usually it's one, two or three races, and those are races where you, as his opponent in the championship, need to capitalise.
"Sebastian [Vettel] couldn't do that in China and that was a big setback for him because Lewis always comes back so strongly."
Hamilton is currently nine points behind Vettel in the drivers' championship.