Sweden's Peter Hanson could miss the Open Championship starting on Thursday because of a recurring back problem.
The Ryder Cup player was forced to pull out during the final round of the Greenbrier Classic in West Virginia at the start of this month.
"With seven holes to go we had a rain delay and then after the delay I couldn't get up and move so I had to withdraw," he explained.
"The big test for me will be tomorrow (Tuesday) because if I can play nine holes pain-free then that will be the key to my playing or not.
"I am a 50-50 chance of teeing up on Thursday. I thought the disc problem was getting better but then it starts to feel a lot worse."
The 35-year-old is due to play alongside Japan's Hiroyuki Fujita and American Bill Haas in the opening two rounds at Muirfield. If he fails to make it Dutchman Joost Luiten is the first alternate.