Tiger Woods withdrew from the WGC-Bridgestone Invitational during Sunday's final round, sparking fears that he had suffered further serious damage to his back.
Playing just his third event since undergoing microdiscectomy surgery on 31 March, Woods had completed his opening eight holes in three over par to lie four over for the tournament when he pulled out on the ninth hole at Firestone Country Club.
The former world number one looked in severe pain after hitting his tee shot on the 494-yard par four, wincing as he struggled to bend down to pick up his tee.
And minutes later he was on a golf cart being driven straight to the car park, where he again looked in real discomfort as he prepared to leave a venue where he has won eight times, including by seven shots last year.
Asked if he had suffered the same injury which led to the operation and ruled him out of the Masters and US Open, Woods told reporters: "It's just the whole lower back.
"It happened on the second hole when I hit my second shot. I fell back into the bunker. Just jarred it. It's been spasming ever since."
On his prospects of playing in the US PGA Championship next week at Valhalla - where he beat Bob May in a play-off in 2000 - the 38-year-old added: "I don't know, (I'm) just trying to get out of here."
Woods then gingerly made his way to the passenger side of his courtesy car before being driven away by caddie Joe LaCava.
The 14-time major winner had made the ideal start to his round with a birdie on the first, but mishit his approach to the third into water, saved par on the fifth despite his tee shot coming up 60 yards short of the green, bogeyed the sixth and then ran up a double-bogey on the seventh.
European Ryder Cup captain Paul McGinley, commentating on Sky Sports 4, said: "He looks like he's maybe come back too early. The way he played I saw him hit shots on the front nine there that I've never seen him in his life come close to hitting. There's obviously something going on."
Woods missed the cut in his first event back at the end of June and then suffered his worst ever 72-hole finish in a major championship as a professional, ending up 69th in the Open at Royal Liverpool despite an opening 69.
He had previously spoken about being "pain-free" for the first time in two years, but said on Wednesday that other people who had undergone the same surgery were surprised he was able to play again so quickly.
"This is only my third event back after back surgery and that's something that I have had to keep in mind. I've been in this situation before and it takes a bit of time," Woods said in his pre-tournament press conference in Akron.
"The knee (injury) is so much easier to come back from. The back injury was way more debilitating than I thought. And most of the people I talked to who have had the procedure have no idea how I'm even back here playing. They just can't understand that."
Woods did joke that the operation had at least one beneficial side-effect, adding: "We've been trying to shorten my swing over the years and the perfect way to do it is just have back surgery. I can't turn that far."
Woods has been drawn to play alongside Phil Mickelson and Padraig Harrington at Valhalla but his participation in the final major of the year looks extremely unlikely.
Masters champion Bubba Watson, who had been playing with Woods, said: "The shot on two is where he might have tweaked it a little bit...and then hitting out of the rough a few times is just going to make the strain on the back even more.
"Hopefully he recovers good and he gets ready for next week. I told him I was praying for him."