While Trey Lyles proved surprisingly effective last season, he wasn't able to replicate his success this season. With Keldon Johnson starting over him, he struggled to carve out a role off the bench and eventually saw his minutes dwindle.
When he did play, he failed to make much of an impact and proved too passive at times. He missed the last 26 games with a sprained ankle, though he had already fallen out of the rotation by then.
With Trey being an unrestricted free agent, it seems highly unlikely that he'd return next season. While he's young enough to fit the Spurs timeline, the team would be wise letting him leave.
After 10 seasons, it seems time for the Spurs and Patty Mills to part ways.
Mills can still definitely play and does hold value to the team both on and off the court. That said, San Antonio should be looking to go younger, and Tre Jones seems to be a capable replacement. While Jones certainly doesn't replace Mills' shooting, he does provide strong playmaking and pesky defense.
The Spurs could also look to play Dejounte Murray and Derrick White more minutes, limiting Patty's role even further. Rather than pay a premium to keep Mills, such a role might be better served for a young guard or another veteran guard.
Losing Patty would be difficult, but at 32, parting ways now could both save the Spurs money going forward and also clear a path for a younger guard.
Gorgui Dieng played well in his short stint with the team and even displayed some intriguing skills. That being said, with Jakob Poeltl and Drew Eubanks already on the roster, Dieng may be hard-pressed to carve out a bigger role, given their solid seasons.
Gorgui chose the Spurs after being brought out by Memphis, but after sharing minutes at backup center, he may look elsewhere in free agency. Though Dieng does provide the Spurs with good depth at center, they can't guarantee him a larger role and should also look elsewhere.
San Antonio could just as easily find a serviceable third center in the draft or free agency, making the decision to move on from Dieng easier and likely cheaper.