Lonnie Walker may have already hit his ceiling
Like Derrick White, Lonnie Walker is in a bit of a precarious spot on the team. He has the tools to become another very good two-way player for the Spurs, and perhaps at some point even an All-Star. He's an elite athlete, can shoot the ball, and plays respectably well when defending the ball-handler.
That said, Walker is also by far the most volatile young Spurs player to the point that he may have already reached his ceiling. While he's an incredible athlete, he's also very right-hand dominant, is still very much a work-in-progress in terms of court IQ and decision-making, and was statistically one of the worst defenders in the league this past season.
The fact of the matter is that he's coming up on a contract year and he will need to show some form of improvement for the Spurs to consider giving him a hefty extension. Otherwise, he could be one of the first young Spurs players to see the door. Fortunately, though, there are a couple of big caveats to the assessment of him above.
First and foremost, it's worth remembering that this past season was Walker's first full season playing a significant role for the team. In the 2019-2020 season, he only played about 16 minutes per game and frequently found himself competing for minutes with Bryn Forbes and Marco Belinelli.
While Walker's first season in an important role looked statistically less impressive than, say, Derrick White's for example, Walker is still two years younger than White was in his sophomore season and is playing on a worse team in a significantly more competitive Western Conference.
Furthermore, I think Walker's negative impact on defense is dramatically overstated. There is evidence that suggests that he had some of the most difficult defensive matchups out of all NBA players this past season. In fact, according to BBall Index, through late February of this past season, he had slightly more difficult matchups than Ben Simmons, a member of the 2020-2021 NBA All-Defensive First Team.
This coming season, Walker will likely roll out in a vastly improved bench lineup that will more than likely feature some of the Spurs' better young defenders in Tre Jones and Devin Vassell. Combined with more practice time compared to last year, I expect the bench unit's team defense to look far more calculated and effective.
Ultimately, I think what may have been barring Walker from looking like a much better player this past season was the quality of personnel he was forced to play with along with a constantly changing role. While I expect to see a big improvement from him though, it's far from being a sure thing.