The rib injury which blighted Paul Casey's season now appears to have ended it - and the Ryder Cup star's bid for a maiden Order of Merit title.

The world number four quit after just two holes of his final round at the World Golf Championships-HSBC Champions because of a recurrence of the nagging muscle problem.

Casey returned from three months out at last week's Volvo World Match Play Championship but appeared to have recovered from his three defeats in Spain after playing his first three rounds in Shanghai in six under par despite insisting he was still not 100%.

But, after also pulling out of next week's Hong Kong Open, the 32-year-old will head to America to consult with his long-time specialist.

A decision on whether he will return for the season-ending Dubai World Championship later this month is expected towards the middle of next week.

He remains fifth on the Race to Dubai with just two events remaining despite failing to improve his position this week, but a prolonged period of rest over the winter appears to be the best option.

With leader Lee Westwood and nearest challenger Rory McIlroy, now second following his fourth-place finish in Shanghai, teeing up off Hong Kong, they have a chance to pull away at the top of the money list - although the prize on offer in Dubai means the race for the Harry Vardon trophy will still go down to the final week.

"He is hugely disappointed about not being able to play Hong Kong," said Casey's manager Guy Kinnings.

"He very much felt he had a chance to win the Race to Dubai, that's why he had been aiming to come back last week and this week and play in Hong Kong."

In 2006, Casey was leading the race to become Europe's number one heading into the final event but was affected by illness at the season-ending Volvo Masters and was ultimately pipped to top spot by Padraig Harrington.

"He has felt pain before, but he felt it severely enough," added Kinnings.

"We always agreed that if he felt that he would stop. He is going to withdraw from next week and get back and see the specialist who he has been working with and get it scanned.

"He is sufficiently concerned to do that straightaway, he doesn't know if this means recurrence and out for a while or just a worrying tweak."