The Spurs Are Underperforming on Defense
Defensively, San Antonio has the personnel to be a top 10 defense but has underachieved mightily on that end. Some of that is due to COVID-19 related absences, which drastically affected the team's rotation.
That said, even when mostly healthy, San Antonio has had games in which they've been blown out after allowing teams to get hot from three. Take last month's game against Denver for example. San Antonio managed to give up 20 made threes on 43 attempts. In fact, the Spurs allow the sixth-most 3-point attempts in the NBA, which is a better indicator of 3-point defense than 3-point percentage allowed.
San Antonio simply allows too many attempts, and good shooting teams can make them pay. When that happens, San Antonio has to rely on a below-average offense to pick up the slack, and that often doesn't happen.
Which Side of the Ball Is the Biggest Problem?
So which one is the biggest problem for the Spurs? I would say it's defense. That may come as a surprise to some, but they should be much better than they've been. I mean, they have seven rotation players that qualify as good defenders but San Antonio is somehow less than the sum of their parts.
While having so many plus defenders may bode well for future Spurs teams, the fact that they have underachieved has effectively made them a lottery team this season. This is despite advanced stats showing that they should be a .500 team, which would be good enough for the 8th seed in the Western Conference.
Ultimately, while San Antonio hasn't been particularly good on either side of the ball, defense is their biggest problem. Again, they should be a lot better than they currently are on that end of the floor given their personnel.
Luckily, going forward, the Spurs' defensive issues should be correctable now that the team is almost fully healthy. As a result, while defense is San Antonio's biggest problem now, they already have the right players needed to eventually solve their issues.