The 32-year-old American star landed "very hard" after a fall down stairs at his rented house in Augusta.
The 32-year-old American star landed "very hard" after a fall down stairs at his rented house in Augusta, his agent said, but still hopes to be fit enough to tee off in Thursday's first round.
Before the announcement sent shock waves through Augusta National Golf Club, Johnson was the oddsmakers favorite to collect the champion's green jacket after winning his past three starts.
"At roughly 3 pm today, Dustin took a serious fall on a staircase in his Augusta rental home," his agent, David Winkle, said in a statement. "He landed very hard on his lower back and is now resting, although quite uncomfortably.
"He has been advised to remain immobile and begin a regimen of anti-inflammatory medication and icing, with the hope of being able to play tomorrow."
Johnson, who practiced briefly Wednesday before storms closed the course early, is scheduled to tee off in Thursday's final first-round group at 2:03 pm alongside compatriots Jimmy Walker and Bubba Watson.
Golf Digest-Golf World reported on its website that Johnson's feet went out from under him when he was on a short staircase going into a basement and that his fiancee, Paulina Gretzky, said the fall "sounded like a huge thud."
"Dude, I can't believe I did that," Johnson told his trainer, according to the report, which also said Johnson has pain in the lower left side of his back below the rib cage.
He expects to visit the PGA Tour medical trailer Thursday morning, possibly having an X-ray or MRI exam.
Even if he is able to start, his trademark long driving skills could be compromised if the back trouble continues, a potentially serious handicap over the 7,435-yard layout.
Johnson averaged a tournament-best 299.5 yards in driving distance last year, when he finished a career-best fourth, and averaged a Masters-low 26.50 putts per round in 2015, when he shared sixth.
"Everything has got to be working this week," Johnson said Tuesday. "If you want to win, you're going to have to drive it well. You're going to have to hit your irons well. You're going to have to putt it well.
"I don't know how many guys are in the field, but they are the best golfers in the world, so you've got to bring your best stuff if you want to win."
The injury marks another setback in the checkered career of Johnson, who won his first major title last year at Oakmont.
Johnson, hoping to make his seventh Masters start, withdrew from the 2012 edition after a back injury suffered, his agent said, when he lifted a jet-ski.
At the 2010 PGA championship, Johnson carried a one-stroke lead into the final hole but received a two-stroke penalty for grounding his club in a bunker, doing so in a sandy area he did not recognize as a hazard.
At the 2010 US Open, he led by three entering the final round but fired an 82 and finished eighth. Johnson was level with Jordan Spieth on the 72nd hole of the 2015 US Open but missed a three-foot birdie putt to force a playoff, settling for second.
Johnson -- born an hour's drive north of Augusta National in Colombia, South Carolina -- became the world number one by winning at Riviera in February and followed with World Golf Championships Mexico and Match Play titles last month. He would be the first player since 1976 to enter the Masters on a three-event win streak.
The last time a world number one missed a major tournament was in 2015 when Rory McIlroy suffered an ankle injury a few weeks before he was to have defended his title at the British Open.
Johnson has not missed a major since the 2014 PGA Championship, which came just after he announced he was taking a six-month break from golf to seek professional help for "personal challenges."
No world number one has won the Masters title since Tiger Woods in 2002 and no player since 1940 has made the Masters his fourth victory in a row.