The arrival of LaMarcus Aldridge in free agency gave the Spurs a huge offensive talent boost, but questions about fit and depth on the defensive end worried experts and tempered what should have been massive expectations.

Eight games into the season, those concerns are starting to fade. San Antonio ranks third in the league in defensive rating, allowing just 94 points per 100 possessions. On Wednesday, they held the Trail Blazers' high-powered attack below their average field goal percentage, three-point attempts per game and their offensive rebound percentage. Portland scored 101 in the Spurs' win, but it was a hard 101.

It looks like the Spurs will continue to be a defensive juggernaut despite on paper not having ideal personnel. Here's how they're doing it.

Tim Duncan and Kawhi Leonard are elite

Aldridge's arrival has turned Duncan into a role player on offense; he's averaging a career-low in field goal attempts and points. The upside to not having to worry about scoring is that Duncan is using more energy on defense.

Duncan is blocking fewer shots than in past years, but opponents are shooting nine percent worse on shots within 10 feet of the basket when he's defending them, per SportVU data. The Spurs as a team allow a low 52 percent on point blank shots when he's on the court, and he allows just 45 percent on shots he defends at the rim, per Nylon Calculus.

Duncan's eating post players alive as well, allowing just four baskets and causing three turnovers in 15 possessions in that situation, according to Synergy Sports stats. Even at 39 years of age, he's still a great defensive anchor.

While Duncan holds down the paint, reigning Defensive Player of the Year Kawhi Leonard wreaks havoc in the perimeter. Despite guarding the opponent's best defensive player, Leonard holds his man to a field goal percentage eight percent lower than his usual average. So far this season, he's held Kevin Durant to 6-for-19 shooting and Carmelo Anthony to 4 for 17.

Not only can he devour presumably unblockable shots like that one, but he strikes fear into ball handlers with his length and quick hands. Consider how hard it is to strip Russell Westbrook while guarding Kevin Durant.

Duncan and Leonard are the backbone of the Spurs' defense and they are carrying out their roles better than ever.

The defensive scheme is smart and everyone follows it

The Spurs have made changes to their roster, but the fundamentals of their defensive scheme remain the same. They don't give up easy layups or threes and instead force opponents into tough, inefficient shots. It continues to work like a charm despite the new personnel.

The Spurs allow the second-highest number of mid-range jumpers per game, trailing only the Bulls. They concede the 10th-fewest corner three-pointer attempts and the second lowest number of three-point shots overall. Only three other teams do a better job not fouling and preventing fast-break points. When opponents get to the rim, there's typically someone there to contest shots.

Teams have to earn points against San Antonio, as their opponent's shot chart for the season shows. There's no green anywhere.

The new additions have all bought into the system and the holdovers can execute the scheme in their sleep. As a result, breakdowns don't happen often, allowing the Spurs to lock down on defense for long stretches, if not all 48 minutes yet.

Gregg Popovich's rotations mask the lack of depth

While the starters have two elite defenders in Duncan and Leonard and two above-average ones in Danny Green and Aldridge, the bench lacks a difference-maker on that end. Manu Ginobili has lost a step, Patty Mills is too small to be a stopper at point guard and Boris Diaw is earthbound. David West is a heady player, but isn't a rim protector. Prospective fourth wing Kyle Anderson has the nickname "Slowmo" for a reason.

The second unit was supposed to hemorrhage points on defense, and when five subs have been on the court together, it has. Lineups featuring Patty Mills, Ginobili, Diaw, West and either Anderson or Rasual Butler have been a disaster in limited minutes.

Fortunately, Gregg Popovich recognized the problem early and has limited the negative impact of the bench's defense through clever lineup management.

Pop recently shortened the rotation to eight main players, reducing the time Anderson and West are on the court. He has staggered the minutes of Aldridge and Leonard to have them on the court with Mills, Ginobili and Diaw, making that the second-most used unit on the team. That lineup has allowed only 93 points per 100 possessions, a mark that would rank second in the league.

The effect is that Aldridge and Leonard are playing heavy minutes, which is ironic for the Spurs. For the first time in years, they are riding their two best players hard. So far, they've been able to handle it, and that's allowed Popovich to keep good defenders on the court in every lineup.

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The Spurs have the personnel, scheme and the right rotations to maintain an elite defense. Aldridge has done a better job than expected on that end, the two starting wings are very good defenders and Duncan is still a very good rim protector. Those four cover for Tony Parker's decline, while Pop's rotations prevent the bench from being a drain for the team.

Late-game execution remains a problem and there are times when the offense struggles to create as many good shots as it did in the past. That's to be expected as the team transitions to Leonard and Aldridge as primary options.

But the defense being this good already is a surprise and the main reason why the Spurs appear to be head and shoulders above Golden State's other challengers in the West.