Dustin Johnson was playing with his son Tatum when he slipped and injured his back
Johnson started the week as favourite to secure his first Masters after finishing fourth last year.
The 32-year-old won his first major last year when he took the US Open in June and has won five more titles since then.
However, the world No 1's chances were dealt a blow when he slipped on three wooden stairs in his rented Augusta home.
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And trainer Joey Diovisalvi has revealed the injury, news of which has seen him drift to third in the betting behind Jordan Spieth and Rory McIlroy, happened during a simple game with one of his children.
"He was playing with [his son] Tatum" Diovisalvi said.
"Life sometimes happens.
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Two-year-old Tatum Johnson was playing with his father Dustin when the accident happened
"He's not angry or upset.
"I'd say more discouraged.
"He told us [last night], 'I'm going to do everything I can to go out and play tomorrow.
"That basically is his attitude."
PGA medical staff said they reacted quickly to ensure Johnson did not wake up stiff or in spasm on Thursday morning.
And manager David Winkle had revealed on Wednesday that he had been given a regime of anti-inflammatories and icing to deal with the injury.
"He landed very hard on his lower back and is now resting, although quite uncomfortably," Winkle said.
Johnson is understood to have travelled to the course this morning and will make a final decision about whether to play after completing his warm-up for the round.
The American is due to tee off with fellow countrymen Bubba Watson and Jimmy Walker at 2.03pm local time or 7.03pm in the UK.