The "Very Likely Safe" Group
The Spurs have historically thrived by having centers that can do exactly what Jakob Poeltl is doing and do it well. The only difference is they don't exactly have the star pieces around their effective center that they used to.
Poeltl is one of the best defensive centers in the league and is only 26. It's easy to see the overall impact he has when he's off the court, and that's a good sign of a likely irreplaceable player. That said, if the Spurs believe they can upgrade soon for a center with more two-way ability, they might just have to take that chance.
As the "older" guy of the core, Derrick White could provide pretty good trade value if the Spurs opted to give guys like Josh Primo and Devin Vassell bigger roles sooner rather than later. That said, I don't see San Antonio making a move like that yet. White is one of the best shot-blocking guards in the NBA and provides a lot even when his shot isn't falling.
Like I said with Primo, the Spurs opting to trade Doug McDermott just wouldn't make a lot of sense. They traded for him for a reason, and his acquisition is paying off in his 3-point shooting and overall efficient scoring. He made it clear he's always wanted to play in San Antonio, and the Spurs should be happy with what he's brought so far.
Where Tre Jones fits in the Spurs' longer-term future might depend on how they end up using Josh Primo once he starts getting regular minutes. For now, Jones is looking like a pretty good backup point guard when he's out there, and I don't think the front office is ready to let him go anytime soon.
Jock Landale's early concussion and COVID protocols slowed his start to the season, but he's been showing what he's capable of lately. The Spurs don't really have any guys his size who can spread the floor like he does, and that's an increasingly important skill in the modern NBA. I don't think there will be a demand from teams for Landale yet, and if there was, it wouldn't be a big offer.