During the 2011 NBA lockout, Thompson attended classes at the University of Texas to finish his college degree. Thompson signed his rookie contract with the Cavaliers on December 9, 2011 before the start of training camp. Thompson made his professional debut against his hometown Toronto Raptors on December 26, 2011. In 17 minutes off the bench, Thompson scored 12 points and pulled in 5 rebounds. Thompson came off the bench behind veteran forwards Antawn Jamison and Anderson Varejão for the first three months of his rookie season but still managed to make his presence known on both ends of the court. His production increased in February as his minutes increased, and he recorded three double-doubles that month. His best performance in February was a 15-point, 12-rebound, two-block effort in 30 minutes of play as the Cavaliers defeated Sacramento 93–92. Both Thompson and teammate Kyrie Irving were later named to the 2012 Rising Stars Challenge. However, Thompson was selected to play for Team Shaq, while Irving was selected by Team Chuck.