Nelson, the team's leading scorer and rebounder last season and an all Pac-10 selection, sat out the last third of practice on Tuesday after complaining of pain in his left foot and ankle. X-rays showed no damage.
"We're not sure what it is," Howland said. "It's not an ankle sprain. It's a part of his foot he feels is giving out on him all of a sudden, so I don't know if it's from wearing a different pair of shoes or what."
Howland said Nelson recently switched shoes.
"I know that's happened to me at times when you (don't know) exactly how you were landing and running, so I don't know if it's something that simple or if it's something that's serious or what," he said. "There was no injury to it where he twisted it or someone landed on it, at least that he remembers."
Nelson, who averaged 13.9 points and 9.1 rebounds last season, had 16 points and 10 rebounds Sunday in UCLA's exhibition victory over Cal State San Bernardino. The Bruins open the regular season Friday night against Loyola Marymount at the Los Angeles Sports Arena.
Peter Yoon covers UCLA for ESPNLosAngeles.com.