Looney "will be out indefinitely," the Warriors confirmed Monday night, following an MRI in the San Francisco Bay Area after he missed the second half of Sunday's 109-104 NBA Finals Game 2 victory over the Toronto Raptors. The costal cartilage connect the sternum to the ribs.
Without Looney, the Warriors will become more dependent on center DeMarcus Cousins, who made a strong return from his own injury in Game 2.
Looney appeared to be dragging his right shoulder in the first half, but it is unclear exactly when he suffered the injury.
Meanwhile, there is optimism among the Warriors that swingman Klay Thompson will play with a mild hamstring strain in Game 3 in Oakland on Wednesday, a league source told ESPN. He will rehab the injury over the next two days.
Thompson, who is officially listed as "questionable" for the next game, exited in the fourth quarter of Game 2 with left hamstring tightness. He said after the game that he expected to play in Game 3, with the series tied at 1.
The results of Thompson's MRI were slightly delayed because the Warriors' charter plane landed later than expected Monday morning, pushing back the process a few hours. The plane didn't arrive back in Oakland until after 4 a.m. local time Monday.
The Game 2 injuries were the latest setbacks for a Warriors group already playing without star Kevin Durant, who continues rehabbing a right calf injury. Meanwhile, swingman Andre Iguodala is dealing with a lingering calf injury.
The Warriors remain buoyed by the fact that they still found a way to win Game 2 and the belief that Durant will be back at some point in this series. Kerr said before Sunday's game that it was "feasible" Durant could participate in a practice, then play in a Finals game.
A second Warriors flight out of Toronto, carrying staff and family members, was scheduled to leave after Game 2 but had a mechanical issue that forced the plane to return to its gate. The second flight arrived safely in Oakland late Monday afternoon. Staff members traveling on the coaches and players' plane said they were happy to have been on the first flight out.
ESPN's Nick Friedell contributed to this report.