Boruc made his debut for Celtic on 2 August 2005 when he replaced David Marshall for the second leg of their UEFA Champions League qualifier tie against Slovakian side Artmedia Bratislava. Celtic won 4–0, but having lost the first leg 5–0 still went out on aggregate. Boruc quickly established himself as first-choice keeper at Celtic, displaying a combination of outstanding shot-stopping, immense self-belief and, at times, outright arrogance. Indeed, gestures allegedly made by Boruc towards Rangers fans at Ibrox Stadium on 12 February 2006 later resulted in the player being cautioned by the police. In his first season, he made 40 appearances and kept 17 clean sheets. Boruc also added to his winner's medals collection as Celtic won the Scottish Premier League and the Scottish League Cup, the latter after beating Dunfermline Athletic 3–0.