“Beware the injured/sick golfer” is an old golfing adage. Other golfers are apt to feel sorry for this golfer—who then destroys them on the golf course.
 
“The Sick Golfer” was a poem by Edgar Guest (1881-1959) that appeared in many newspapers in 1923. “The old adage that ‘A sick golfer is the hardest to beat’” was cited in print in 1933. “‘Ware Sick Golfer” was cited in print in 1937.
 
 
13 October 1923, Boston (MA) Herald, “Just Folks” by Edgar A. Guest, pg. 7, col. 5:
THE SICK GOLFER
I shuddered when I heard him say:
“I am not feeling well today.”
I hoped he’d not propose a bet,
I’ve never licked a sick man yet!
 
28 February 1933, Chester (PA) Times, “Gene Sarazen Annexes TItle,” pg. 14, col. 4:
MIAMI, Fla.,Feb. 28—(INS)—Olin Dutra of Santa Monica, Calif., P. G. A. champion, who was awarded by Gene Sarazen 11 up and 10 to play in their 72-hole “unofficial world’s golf championship” match that ended here yesterday, ruefully recalled today the old adage that “A sick golfer is the hardest to beat.”
 
16 May 1937, Seattle (WA) Sunday Times, “Tee Talk” by William F. Steedman, pg. 20, col. 4:
“Ware Sick Golfer!
(...)
And Pursey muttered, as he turned in the door of his shop, “Never again will I give strokes to a sick golfer.”
 
5 April 1950, The Times Recorder (Zanesville, OH), “‘Cripples’ Watched In Masters Golf Tourney” by Sterling Slappey, pg. 18, col. 4:
AUGUSTA, Ga., APril 4.—Beware of the sick golfer in the Masters tournament.
 
14 July 1982, The Capital TImes (Madison, WI), “Flu hones Niklaus’ shots ad British Open start nears,” pg. 49, col. 1:
TROON, Scotland (AP)—Jack Nicklaus hopes the old bromide of beware of the sick golfer holds true for the 111th British Open beginning Thursday.
 
Google Books
A Game of Sudden Death
BY Douglas Rutherford
Leicester: Ulverscroft
1990, ©1987
Pg. 187:
“There’s an old adage, Bryan. Beware the sick golfer.”
 
19 January 1998, Pacific Stars and Stripes (Tokyo, Japan), “Golf Roundup” (The Associated Press), pg. 29, col. 3:
“Beware the injured golfer,” she said Friday after posting a 3-under-par 69 for a one-stroke lead.
   
Sports Illustrated
September 20, 1999
Money Players
Alan Shipnuck
(...)
Beware the injured golfer.
   
Google Books
Kathy Whitworth’s Little Book of Golf Wisdom:
A Lifetime of Lessons from Golf’s WInningest Pro
By Kathy Whitworth with Jay Golden
New York, NY: Skyhorse Publishing
2007
Pg. ?:
THERE’S A SAYING, “Beware of an injured golfer.” Many times it’s true. They feel so bad, they just have to play and survive.
   
CNN.com
Adam Scott: Beware the sick golfer
March 21, 2014—Updated 1246 GMT (2046 HKT)
(CNN)—There’s an old saying in golf—“beware the injured golfer.”
 
Well, on this occasion it was the sick golfer who came out on top on the opening day of the Arnold Palmer Invitational at Bay Hill, Orlando.
 
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Golf Digest
‏@GolfDigestMag
“Beware the injured golfer,” goes the old saying. After yesterday’s win, the phrase has a new meaning for Rory: http://golfdig.st/1r78Mwe
7:51 AM - 26 May 2014