Golf's current and former superstars were on vastly different trajectories for the year's final major tournament with McIlroy going for a hat trick of triumphs and Woods' status in doubt a day after a painful exit at the World Golf Championships event in Akron, Ohio.
McIlroy won his third major title last month at the British Open and took his first WGC crown Sunday at Akron, arriving at Valhalla as a clear favorite and the first player since Woods in 2009 to enter a major after back-to-back wins.
"It's great to have a chance to try to go there to try to win three in a row," McIlroy said. "(It's) one I desperately want to win."
McIlroy overtook Australian Adam Scott for the world number one ranking after winning Sunday.
"My goal now until the end of the year is just to try to win as many golf tournaments as I can. It's not to try to finish number one," McIlroy said.
"I feel like with the way I'm playing there's a few left in me this year."
Woods was supposed to be practicing Monday at Valhalla, where he won the 2000 PGA title, but instead was in Florida having his back examined while the course was examined by caddie Joe LaCava, who had to help Woods put on his shoes in the parking lot after he withdrew from the final round in Akron with back spasms.
Mark Steinberg, Woods' agent, told ESPN Monday that the star's ability to play might not be decided until the eve of the tournament.
"Way too early," Steinberg said in a text message. "He has to rest and get treatment and then assess later... Nothing further today, maybe (not) even tomorrow."
Woods, a 14-time major champion chasing the record 18 won by Jack Nicklaus, underwent back surgery March 31 to ease a pinched nerve and returned in Washington in late June, missing the cut at the US PGA event his foundation owns.
After posting his worst 72-hole result in a major by sharing 69th at the British Open, Woods "jarred" his back by hopping into a bunker after hitting his second shot Sunday at the second hole at Firestone.
Playing through pain, Woods lasted until hitting his tee shot at the ninth and then walked off.
"It's just the whole lower back," Woods said.
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That left Phil Mickelson, who fired his low round of the year Sunday with a 62, not knowing whether he will be playing alongside Woods or not in the first two rounds.
"It didn't look good. It looked like he was really in pain," Mickelson said.
"I hope he's OK. I hope he's able to play. I hope it's a muscle and nothing serious because I'm really looking forward to playing with him.
"As much as I love playing with him, playing against him, trying to beat him, we all want him in the field. We all want him back. I just hope he's OK."
Reigning Masters champion Bubba Watson went back to Woods' most recent major victory, on a broken leg at the 2008 US Open, to cite his ability to cope with pain and pressure and still win.
"This man won a tournament with a hurt leg, so he's going to try to be a champion, try to tough it out," Watson said. "Obviously, it got too tough for him.
"He was brought up a military man just like myself and you try not to show it."
That leaves the stage to McIlroy, who won his first major at the 2011 US Open and added the PGA Championship in 2012.
"Obviously, Rory's in incredible form at the moment," Scott said. "He'll be the man to beat by the looks of things, and I'll be gunning for him for sure."
McIlroy averaged a PGA season-best 334 yards off the tee last week and ranked 12th in driving accuracy while leading in greens reached in regulation as well.
That enabled him to hold off Spain's Sergio Garcia down the stretch as he had at the British Open.
The 34-year-old Spaniard, seeking his first major title, has finished second in his past three starts and has six top-four US PGA showings this year without a victory.
"Sergio has been playing really well, and he has come close a few times," McIlroy said. "I think another win is right around the corner for him as well."