OAKLAND, Calif.—Earlier in this NBA season, when the Knicks took a drubbing like the one they took here in California on Wednesday, losing to the defending champion Warriors by 36 points, fans could usually take some solace in the fact that rookie Kristaps Porzingis looked like a star in the making, seemingly progressing with each game.

Even in a 95-78 blowout loss to Miami on Nov. 23, for one example, he scored 20 points, grabbed 14 rebounds and blocked two shots. But now, after a 1-for-11 shooting performance in Golden...