(CNN) -- World number three Paul Casey is out of this week's Barclays event in New Jersey, the first leg of the U.S. Tour's FedEx Cup play-off series.
Paul Casey is still suffering from the rib injury which forced him to miss the U.S. PGA tournament.
But Casey, suffering a rib muscle injury, hopes to return at the Deutsche Bank Championship in Boston next week.
Three times a tournament winner this season, 32-year-old Casey picked up the problem during a practice session before the British Open at Turnberry last month.
He finished 47th there, but then had to pull out during the first round of the WGC-Bridgestone Invitational three weeks ago and missed the final major of the year, the U.S. PGA.
"It has been a really frustrating few weeks and, of course, I was particularly disappointed to miss the PGA Championship," Briton Casey told Press Association Sport.
"I had hoped to be back out on the Tour this week. However, I feel pretty lucky that it wasn't more serious and that the injury has responded to treatment so well.
"I am really hoping that it continues to improve and I will be teeing it up in Boston next Thursday."
Casey is still undergoing a course of intensive treatment. It includes having a cortisone injection to the intercostal muscle to assist the recovery of both a tear and a strain.
Casey, who eight days ago lost his lead on the European "Race to Dubai" money list to German Martin Kaymer, lies 18th in the FedEx Cup standings.
Meanwhile, last season's European Tour number one Robert Karlsson, who has been out of action since May, will not be returning for at least two more weeks.
The Swede has been suffering from a blister behind his retina, thought to be stress-related, and has his next doctor's appointment is on September 3.
Having missed the last three majors, Karlsson played the pro-am before the SAS Masters in Sweden a month ago, but was still suffering problems with his depth perception.