We’ve seen this before. A hot shooter pulls up at the top of the key, given plenty of room, and drills the winning shot as time expires. The remaining defenders could have helped by closing the paint but they were standing still, out of the way and unable to help if they wanted to.
During the preceding the team should have discussed how they would guard a game-winning shot attempt and prioritized who should stay on their check and who should help. I think the team should have focused on Kyrie Irving and Wayne Ellington and forced Dion Waiters and Shaun Livingston into the corners. As time wound down, Irving’s options became limited and the defense should have adjusted to take more of them away.