This is something to be worried about to be sure but there are a few things to remember.
First, no one except Parker is averaging more than 30 minutes per game so far this season, and Parker is right at 30. Some of the starters play a bit more against the better teams but not a lot more.
Second, traditionally the Spurs have used the first part of the season to experiment with lineups and to giving the bench players an extended look.
Third, despite the above, the Spurs are only one loss behind the league leaders.
It seems to me that the Spurs are just coasting right now.