England's David Howell carded a 10-over-par 82 on Sunday to slump to a dismal nine over for the tournament.
But after his USPGA horror show the 31-year-old revealed he injured his shoulder whilst warming up.
"I felt my shoulder on my last few shots on the range and thought nothing of it," said the world number 10.
"But it soon became a problem and I thought about withdrawing with five to play. I don't like to blame adversity, but clearly it was affecting me."
Howell, who is also struggling with a knee injury, currently leads the European Ryder Cup points race and any doubts over his fitness will be a huge concern for captain Ian Woosnam.
"I'll go and get treatment - I'm in the wars"
He missed two months of the season last year after straining a stomach muscle and missed another month this year with a bad back. Howell's round on Sunday was the second worst 18-hole effort of the week at Medinah. Only American veteran Jay Haas, with an 83, did worse than the Swindon-born golfer.
Despite his fitness worries, Howell said he would play as planned in next week's World Golf Championship event at Firestone, Ohio.
"I'll go and get treatment. I should at least start (the first round on Thursday) because I need to get in my 15 starts over there," he said.
That is the minimum number of events a player must compete in to keep his US Tour membership, but the bigger concern, obviously, is the Ryder Cup next month.