Shooting Guard: Derrick White
While Derrick White has experience coming off the bench, I expect him to start at shooting guard again next season. After all, with DeRozan gone, San Antonio will need White as much for his ball-handling and play-making as his potential to score. In terms of proven scoring ability, he probably has the best chance of being a consistent contributor in the starting lineup.
He's flashed the ability to help carry the offense before, like in the NBA bubble, where he averaged an impressive 18.8 points, 4.6 rebounds, and 5.1 assists per game. It's certainly possible that he'll be able to replicate that production, or perhaps even improve upon his play assuming that he's able to stay healthy.
White will also play just as big of a role on defense. With Murray and White both starting, the Spurs should be able to effectively defend just about any opposing backcourt. He, in particular, has shown the ability to lock down opposing players while also providing help defense by blocking shots, forcing turnovers, and taking charges. Overall, White's two-way ability is vital to San Antonio's success next season.
While he may not necessarily have the highest potential of the Spurs' long-term options at shooting guard, he'll likely have the most significant short-term impact on both ends of the floor, making him the team's best option in the starting five next season.