The Blazers were without All-Star guard Damian Lillard but still put up little fight against a Spurs team which came into the contest six games below .500.
"My biggest concern, I think, at the moment, is I want us to compete harder, man. I want us to compete harder,'' Billups said. "I want us to be competitive in every game. And I don't feel like every night we do that. We don't. And it concerns me. And I've felt that way in a lot of our wins. This is not just after a loss, me saying this."
The Blazers fell below .500 with the loss and will be without their superstar for the near future. Portland announced on Wednesday that Lillard will miss 10 days with lower abdominal tendinopathy before being reevaluated.
They could be missing third-year guard Anfernee Simons soon as well. Simons, who started in Lillard's place on Thursday, suffered an ankle sprain in the first quarter and did not return.
The Blazers hired Billups this offseason with the hopes of improving the team's defense, which has struggled again nonetheless this season. The Spurs, who have the fewest 3-pointers in the NBA, made 11 in the opening half -- including the first three baskets of the game.
"We didn't have enough personal pride defensively in the game tonight," Billups said.
The loss is just Portland's second at home this season. They are 1-10 on the road.
Information from The Associated Press was used in this story.