Spanish Grand Prix
Venue: Barcelona Date: 7-9 May Sunday, race coverage: 1300 BST - BBC One, Red Button, Radio 5 live, online & mobiles
BBC coverage in detail
By Sarah Holt
BBC Sport at the Circuit de Catalunya
Rosberg is currently second in the drivers' championship
Nico Rosberg says Mercedes have gone backwards after introducing major upgrades for the Spanish Grand Prix.
Rosberg was 1.413 seconds off Mark Webber's pole time and for the first time this year was out-qualified by team-mate Michael Schumacher.
"This weekend has to be a setback for us," said Rosberg, who starts eighth on the grid, two places behind Schumacher.
"I came here thinking we could really put on a challenge but it hasn't worked out. We have made a small step back."
Mercedes have introduced major aerodynamic upgrades to their car and lengthened the wheelbase to boost performance.
Both drivers had complained of understeer - a difficulty in getting the car turned into corners - and it was hoped the modified car would better suit Schumacher's driving style by redistributing the balance.
Rosberg, who is defending second in the championship behind McLaren's Jenson Button, admitted Mercedes had improved the car in absolute terms, but said he was concerned that Mercedes had slipped back in comparison to the other top teams, Red Bull, McLaren and Ferrari.
"We have definitely made a step with the car," Rosberg said. "However, there must be some sort of unknowns.
"Michael's performance in Shanghai, we still don't know why that was so poor. And if I am to compare myself to Red Bull it's a bit strange I was 0.4secs in Shanghai and now I am 1.4secs away.
"We definitely need to look into it and do some analysis as to why that is the case."
Veteran Schumacher went some way to answering his critics by finishing 0.114 seconds ahead of Rosberg, out-qualifying him for the first time since his comeback.
The 41-year-old said he preferred the modified Mercedes but conceded there is still work to be done if the team, who won the 2009 titles as Brawn, are to get back to the front.
"I feel the car a lot more in my hands," said Schumacher. "I can work the car certainly better than I have been able to.
"I'm not sure it is so much progress. I'm not particularly happy to be more than a second off the Red Bull.
"From where we are we have to look forward and we have to understood what it is we have to do and to react to close the gap.
"But Barcelona is a track where any aerodynamic efficiency pays a lot more in proportion to other tracks like Shanghai and it might be a completely different picture coming to Monaco."