Cliff-Robinson also committed two fouls, and Cummings was due for a penalty shot.
Beelman sent in the veteran Mychal Thompson to play the final two rounds.
As Hornacek hit a buzzer-beating catch-and-shoot jumper at the end of the second quarter, the Trail Blazers led 58:52 at halftime, ahead by six points.
The rookies on the Trail Blazers finally experienced what NBA competition and game pace were all about.
The previous three games had been a breeze, with them playing almost leisurely for three-quarters to get a feel for the atmosphere.
The defense of the Spurs was first-rate in both hardness and focus; by halftime, only Gan Guoyang managed to play with ease.
He scored 18 points, assisted 5 times, and grabbed 11 rebounds, defending the Trail Blazers' three-second zone while also doing a lot of auxiliary work.