Barcelona defender Jeremy Mathieu has admitted that he was "surprised to play left-back" in Saturday's 3-1 defeat to Real Madrid.
The Frenchman, who joined Barca from Valencia in the summer, has mainly been deployed as a centre-back this season, but started on the left side of defence in El Clasico.
"I was surprised to play left-back, but the coach decides," Mathieu told a French radio station.
"I thought we did well in the match but we were exposed to the counter-attack, which is Madrid's great strength.
"And it's true, the second goal really hurt us. Even though we have big players like [Gerard] Pique, Sergio Busquets or myself, we still have problems with set pieces. We have to work harder."
Prior to Saturday's match, Barcelona had not conceded in the league this season.
No Data Analysis info
Germany
Switzerland
Hungary
Scotland
Spain
Italy
Croatia
Albania
England
Denmark
Slovenia
Serbia
Austria
France
Netherlands
Poland
Romania
Belgium
Slovakia
Ukraine
Portugal
Turkey
Georgia
Czech Republic