As has been discussed at length at ST and here, USG doesn't measure time on ball - it only measures how frequently a player finishes a play with a FGA, FTA or TOV.
What you're looking for are possession stats which you can find here:
https://www.nba.com/stats/players/touches?CF=TEAM_ABBREVIATION*E*SAS&SeasonSegment=Post All-Star&dir=D&sort=TOUCHES
We can look at Pre-ASB v Post-ASB and see that Castle is handling the ball considerably less (which is also apparent by just watching the games). Post ASB you can tell the Spurs have made a point to not have the ball get stuck in ISO dribbles like it was to start the season. Castle is moving the ball a lot quicker and Fox's Time of Possession per game is now ahead of Castle's. Both of there ToP has fallen though, in part in the rise of Harper's and in part because they are just moving the ball faster overall.
| Pre-ASB | Post-ASB |
| Castle Touches per Game | 69.9 | 56.8 |
| Castle Time of Possession | 5.2 | 3.9 |
| Castle Avg Seconds per Touch | 4.45 | 4.18 |
| Castle Avg Dribbles per Touch | 3.76 | 3.37 |
| Castle Pts per Touch | .237 | .295 |
| Fox Touches per Game | 69.2 | 57.8 |
| Fox Time of Possession | 5.1 | 4.1 |
| Fox Avg Seconds per Touch | 4.38 | 4.22 |
| Fox Avg Dribbles per Touch | 3.93 | 3.79 |
| Fox Pts per Touch | .281 | .310 |
| Harper Touches per Game | 45.3 | 50.8 |
| Harper Time of Possession | 3.3 | 3.7 |
| Harper Avg Seconds per Touch | 4.32 | 4.33 |
| Harper Avg Dribbles per Touch | 3.88 | 4.03 |
| Harper Pts per Touch | .240 | .241 |