Werder Bremen defender Per Mertesacker is confirmed out of the Bundesliga start after undergoing a knee surgery. Mertesacker went under knife on Wednesday night and manager Thomas Schaaf announced that the operation was a success. He sustained the injury during the team's training on Tuesday. The German international center-back will be sidelined for up to four weeks. The Bundesliga season starts mid-August. In June, Mertesacker extended his contract with Bremen till 2012.