Red Auerbach once said “An acre of performance is worth a whole world of promise”.
While I don’t know the exact context it was said in, I can use my imagination. Sometimes, we can see a world of promise in someone, and for whatever reason, the performance never comes. This is just the way the world is, particularly in the NBA. Danny Ainge and company do years of scouting on players, whittling down their selections, until, on draft night, they pick someone based on promise.