Woods flew home to Florida on Sunday to be evaluated. It was not clear when he would know if he could play at Valhalla, where Woods won the PGA Championship in 2000. His caddie was on the course Monday.
Woods, in only his third tournament since returning from back surgery, said he jarred his lower back when he hopped into a deep bunker after playing an awkward shot from the slope of the bunker on the second hole at Firestone.
He tried to keep playing and hit some shockingly bad shots one into the water on No. 3, another on the par-3 fifth that was 65 yards short of the flag.
"I watched him play almost the entire front nine," Watson said. "And the shot he had at 5 ... I said, 'Something's wrong with Tiger.' You don't hit that terrible a shot ever ever. And then, of course, No. 9, he just couldn't go any farther. It concerns me because that's an injury that seems like he tried to address before and he's come back from that particular injury."
Watson said he sent Woods a text to tell him he was sorry about the injury, and "I hope you get well soon."
Woods missed two majors and three months of the season after March 31 back surgery. He has no chance of making the Ryder Cup team unless he wins the PGA Championship. If he can't play this week, that would be the end of his PGA Tour season until after the Ryder Cup.