Aldridge's status was unknown after X-rays on his back were negative. He was taken to the locker room and X-ray room in a wheelchair.
Portland opened the third quarter on an 8-0 run after trailing by as many as 17 in the first half but lost Aldridge 43 seconds into the quarter.
Aldridge drove the lane and rose for a running 6-foot jumper but tumbled backward after having his right knee hit Aron Baynes' side. Unable to brace himself after the collision with the Spurs forward, Aldridge took the brunt of the fall on his left hip. He remained on the court for about three minutes, grabbing his lower back before being helped off by teammates.
"When a guy takes a tough fall like that, when you're airborne, you just don't know what can happen when you land," Portland coach Terry Stotts said. "The longer he stayed down, you just don't know."
With Aldridge unable to continue, Thomas Robinson replaced him and made the ensuing free throw.
Aldridge finished with 14 points and four rebounds in 21 minutes.
Information from The Associated Press was used in this report.