Coach Peter DeBoer said the center has an infection in the knee and nothing worse.
"Nothing structurally. It looks like an infection, put him on antibiotics," DeBoer said, according to The Mercury News. "He should be fine."
The 39-year-old Thornton flew to San Jose Saturday while the rest of the team departed on an East Coast swing.
Thornton, who had experienced swelling in the knee, was limited to 47 games last season with knee issues before he underwent surgery in January.
DeBoer termed the infection diagnosis "great news."