1. Paul George
This is a pretty easy choice. Paul George had blossomed in the absence of Granger with his knee issues and became an All-Star. He was the star player during the Pacers' back-to-back runs to the Eastern Conference Finals in both 2013 and 2014.
Then came Team USA in 2014. George was playing with the team in a scrimmage as they geared up for the Olympics and the World Championships. George went to block James Harden's shot and came down on the stanchion holding up the basket and shattered his leg.
George left on a stretcher with a compound fracture and missed almost the entire following season. He wound up playing just six games that year and the Pacers missed the playoffs by a single game. That was when the Pacers' title window closed for good.
Had George not gotten hurt, the Pacers could have made one more run at the title in 2014-2015. The Big 3 in Miami had broken up as LeBron had gone back to Cleveland. The Pacers could have won the East and gotten to the NBA Finals, even without Lance Stephenson as the starting shooting guard. He left in the offseason.
Instead, the Pacers haven't been true title contenders since. George eventually left the Pacers and is now in LA. Some fans want him back in Indy, but it doesn't make a lot of sense to do so financially. He would need to take a pretty big pay cut in order to do so.
Had things gone differently in each of these cases, perhaps the Pacers could have won their first NBA championship. Instead, they were snake-bit with injuries at the wrong time. They are still chasing that first Larry O'Brien trophy.