The Lakers averaged 93.4 points against the Spurs.
The Lakers averaged 93.8 points against the Celtics.
I give the Celtics props for their defense but I believe it's getting overhyped at this point. The Celtics played desperate defense from the first preseason game to the last Finals game, which definitely is worthy of praise. On the other hand, the Spurs didn't even start trying to play defense until maybe around April 1. With four championships in the bag, it's not going to be possible to match the desperation of a team that's built around a bunch of championship starved players. So with that in mind, I don't really think the regular season stats come into play as much as they would if all factors were even.
The Celtics had defensive breakdowns in the playoffs numerous times. The Spurs had some defensive breakdowns but most of their ills were a result of their offense.
The real difference between the Spurs and the Celtics against the Lakers was offense. The Celtics averaged 102.2 points per game against the Lakers. The Spurs averaged 88.4 points per game against the Lakers.
I have little doubt that the Spurs can reclaim their spot as the best defensive team in the league with a few adjustments this summer. Finley was the worst defensive player on the team this past year. Eliminating him will make the team a better defensive team right away. Barry was playing on one leg, so he wasn't much better. On top of that, if the Spurs can have a second bigman that can alter shots in the lane, that'd also be very nice. Those two adjustments, in addition to a defensively adequate backup point guard, and the Spurs defense will be fine.
Offensively, the Big Three has to be healthy and the Spurs need to add a fourth scorer and a few other offensively capable parts. It won't be too difficult to improve on having Finley as the team's fourth scorer.
Overall, the Spurs' postseason demise I'd say was about 90% offensive related -- and a lot of that was due to Manu's injury. Taking out Manu's scoring was bad but taking out the easy shots Manu creates for others might have been even more damaging.
Next year, add back TD and TP playing at a high level, a healthy Ginobili and a couple more offensive parts and the Spurs will be able to make a good run at a le. They weren't too far away this year and they have the means to get the missing parts.
All in all, the Spurs aren't in a bad position. Hopefully they make the right moves in the draft and free agency to give themselves a chance.