CHICAGO -- At this point, LaMarcus Aldridge doesn't have much of a choice rather than to be a focal point of the San Antonio Spurs' offense.

It's a role he filled earlier in his career and it's one he's comfortable playing in the present. But in the times when their offense isn't running as effectively as coach Gregg Popovich would like, the Spurs' defense can help fill in the gaps.

Aldridge scored 28 points and had 10 rebounds, and the Spurs scored 23 points off of turnovers in an 87-77 victory over the Chicago Bulls on Saturday night.

The Spurs, who are playing without last season's leading scorer Kawhi Leonard, led by as many as 17 points in the fourth quarter after a first half in which Popovich said his team "walked through things."

That changed in the third quarter, when San Antonio picked up its defensive intensity and forced the issue against a young Bulls team playing without several key players.

Aldridge continued to be the centerpiece on offense as he picks up the slack of the injured Leonard, will still be out for several weeks with right quadriceps tendinopathy.

"(Leonard) is our main guy and so now that he's out, I'm just trying to help carry the offense a little bit until he's back," Aldridge said.

Aldridge did that and more Saturday night.

"L.A. was just a difference maker for us with the way he scored," Popovich said. "The defense in the second half and L.A.'s offense was what (made the difference)."

Rudy Gay scored 12 points off the bench and Danny Green added 10 for San Antonio.

Robin Lopez scored 16 points for the Bulls, who are without a handful of key players because of injury or suspension while in the midst of a rebuilding effort. Justin Holiday scored 15 points for Chicago while Jerian Grant and rookie Lauri Markkanen each added 13.

Green's 3-point field goal with 5:55 remaining in the third quarter gave the Spurs a 57-49 lead, their largest until that point. The Spurs, who also got 10 rebounds from Dejounte Murray, pushed the lead to 12 on an Aldridge basket and led 65-55 to start the fourth quarter.

After a sluggish start, a solid third quarter made all the difference.

"I felt like we were kind of following (the Bulls) around early," Aldridge said. "But guys got up on the ball (in the second half) ... and we made them try to go through us."

Added Popovich: "We couldn't find much offense so you have to search, especially on the road. We have no excuse for not playing good enough defense. That allows you to stay and play in the ballgame."

For the second straight game, the Bulls remained close through the first quarter and trailed by only three points at halftime.

But with so many key pieces missing -- including forward Nikola Mirotic, who is out 4-to-6 weeks with two facial fractures he suffered in an altercation in practice with Bobby Portis. Portis was suspended for eight games even before a season that was already expected to be filled with growing pains started.

In addition to the 20 turnovers, the Bulls allowed the Spurs to grab 14 offensive rebounds, which gave San Antonio a big edge.

"You cut those down -- it's been two games in a row with the turnovers -- and that's what kills you," Bulls coach Fred Hoiberg said. "A team goes on a run, they get downhill on you, they get easy (baskets) and all of a sudden, you're in a big hole.

"We obviously did not shoot the ball well today, but I'm proud of our guys for sticking in there and giving ourselves a chance on the defensive end."

NOTES: Spurs G Tony Parker is expected to return to five-on-five workouts in about a week after undergoing surgery on his left quadriceps tendon. Coach Gregg Popovich said Parker has been participating in two-on-two and three-on-three drills at full speed and is expected to return in December. ... Parker, along with F Kawhi Leonard, F Matt Costello, and G Darrun Hilliard, were inactive Saturday. ...Bulls F Bobby Portis publicly apologized before Saturday's game for punching F Nikola Mirotic in practice this week. "I'm wrong for what I did," Portis said. Portis served the second contest of an eight-game suspension Saturday. ... Bulls G Zach LaVine (knee), G Cameron Payne (foot) and G Kris Dunn (dislocated left index finger) were inactive.