"He couldn't put his pants on," coach Gregg Popovich said. "It's a little better now than it was this morning."
Keith Bogans replaced Ginobili in the starting lineup.
Ginobili scored 28 points Sunday night in a win at Boston, and has carried the Spurs since Tony Parker broke his right hand. Ginobili replaced Parker in the starting lineup and has averaged 24 points per game while leading the Spurs to an 8-4 record.
Being healthy has been a big reason. The 32-year-old missed the end of last year with a bad ankle and several games this season with a strained groin, but Ginobili has recently said he's felt his best in a long time.
Ginobili went to the bench Sunday shortly after tweaking his back in the final minutes.
He then headed to the locker room before the end of the 94-73 victory.
Ginobili was making his 12th straight start since Parker went out.
"Manu's been playing great for the last month. He's basically taken over the team," Popovich said Sunday night. "He's been the same Manu we've had when we won championships. Without Tony, it's really important for somebody to step up, and he's done it."
The Spurs won back-to-back games against the Cavaliers and Celtics; their only losses in the last month were to Cleveland, Atlanta, Orlando and the Lakers.
Ginobili was a starter when the Spurs won the 2005 NBA title, and for their '03 and '07 championships he was one of the league's top reserves. He was coming off the bench this year until Parker was injured March 6.
Information from The Associated Press and ESPN The Magazine's Ric Bucher was used in this report.