The 31-year-old Queenslander, a runner-up at the 2011 Masters, needed treatment just off the second tee from a physical therapist to help him get through the painful opening round over the formidable Augusta National layout.
Day, who won his only major title at the 2015 PGA Championship, had said an MRI diagnosed lower spinal disc issues in his back.
“If it starts to hurt, I’ll have to tough it out,” Day said earlier this week.
After suffering the mishap on the putting green, Day opened with a bogey and lay on the ground to be treated under the shade of the Georgia pines after teeing off on the second hole, which he went on to birdie.
Day, ranked 14th, added birdies at the par-5 eighth and 13th holes and another at the par-3 16th before a bogey at 17 and a closing par completed his long walk around the famed course.
Day, who took his 12th and most recent US PGA victory last May at Quail Hollow, told the PGA Tour, “All will be good tomorrow.”
Australian Adam Scott, the 2013 Masters winner who opened on 69, is set to partner with Day in a two-man PGA event in New Orleans. He hopes for a speedy recovery for Day.
“Part of me thinks that that’s nothing new,” Scott said. “He knows what to do. I mean he seems to have a bad back all the time and plays great. So I’m sure he knows what he’s doing. Hopefully, it’s nothing major and he gets through and then he can play the rest of the week.”
Day withdrew at Bay Hill last month but took heat on social media after being spotted with his family the next day walking around at Disney World.
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