"I am very confident that I would have won the race," the German said as Mercedes team mate Lewis Hamilton celebrated a home victory that had looked unlikely after he qualified sixth on Saturday.
"I was very much in control until then."
Rosberg had led the re-started race from pole position until the gearbox problem struck on lap 20 when Hamilton was still chasing to catch up.
Despite the setback Rosberg will still lead the championship to his home German race at Hockenheim next week but that was little consolation for the points lost.
"It's difficult to find so many good feelings today but as of tomorrow I will try and look for them again," Rosberg told reporters.
"And yes, there is that fact that I am in the lead of the championship going to a home race."
Rosberg had finished first or second in the eight previous races this season while Hamilton, who has now won five times in the campaign to Rosberg's three victories, had suffered two retirements.
"It was just a reliability problem and a pity," said the disappointed German. "So now we need to review it and push on and try and keep on working on any small reliability issues that we are having.
"Things were going well until that point. I just accept it as a reliability thing and go for the next one, Hockenheim."
Rosberg said he had noted something before the start that felt odd and then after his first pitstop the gearbox deteriorated.
"It resulted in occasions where it went into neutral so I had to pull the clutch and go again. And then it just got worse and worse," he said.
"I had the mentality of OK, let's stop racing Lewis and just try and get this thing to the end... but since it was mechanical there really wasn't anything that could be done from our opinion."
(Reporting by Alan Baldwin, editing by Keith Weir)
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