The German qualified on pole position but he tumbled down the order in the race to finish ninth, as he was not able to look after his rear tyres as much as his rivals.
With the Sakhir weekend having highlighted the key area that Mercedes needs to improve, Rosberg is under no illusions about the hard work that he and his outfit now face.
"[In terms of] the way we manage the tyres and the tyre temperatures, we took a step in the wrong direction and didn't get the best out if it," said Rosberg on his official video blog.
"Altogether, from first to ninth was unreal. I can't believe it. This sport is tough sometimes.
"As nice as it was on Saturday with the pole, it was a terrible Sunday. But there we go.
"The great thing is we have a great car. We just need to sort out these tyre issues, which is easier said than done.
"It is a massive mission and it is the same for everyone. Some people are doing a better job so we are pushing hard for that. "
Rosberg had gone in to the race well aware that he would struggle to keep his advantage at the front, but even he admitted that he did not expect it to go wrong as much as it did.
"We had a great start and lots of fighting, but I knew from corner three onwards I could see the guys behind were putting so much pressure on that I could not go their pace," he said.
"It was tough and I was disappointed as I wanted to be on the podium or fight for the win, or whatever, but to go backwards? It is not great, definitely not great."
Team principal Ross Brawn had said before the weekend that the Bahrain GP would give Mercedes an indicator of if it could be a genuine title contender this season.