Castaignos was featured throughout preseason, most notably scoring the opening goal in a friendly against Celtic on 30 July 2011. He made his competitive debut for Inter Milan in a Supercoppa Italiana, coming on for Joel Obi in a 2–1 loss against Milan, and played in front of 70,000 people at Beijing National Stadium, Beijing China PR. However, Castaignos was controversially dropped from the UEFA Champions League squad. This led the club's chairman Massimo Moratti commenting on the situation on the player, saying: "I was very sorry for Castaignos because he is a great guy, if this story came up he would play him. It's pretty bad, but it's not like you hit someone. We just have to avoid these things." Despite hops of his inclusion in the squad for the tournament, Castaignos remained out of the squad. He made his full league debut, starting a match before being substituted at half time, in a 3–1 loss against Novara on 20 September 2011. Castaignos then scored his first goal in Serie A in an away match to Siena on 27 November 2011, latching on to Thiago Motta's through ball before firing low past Brkić, which proved to be the game winner. However, Castaignos found his first team opportunities limited, due to competitions in the striker positions.