Westwood has won one tournament this year - the St Jude Classic
Germany's Martin Kaymer could prevent Englishman Lee Westwood from taking over at the top of the world rankings.
Westwood will not play for a month to recover from injury - a move that was set to mean he would move ahead of Tiger Woods as world number one.
But Kaymer, who has won his last three events, will play at the Andalucia Valderrama Masters in Spain and could still overtake both Westwood and Woods.
Kaymer is fourth in the rankings after winning the Dunhill Links Championship.
Westwood could have become world number one had he finished in the top two at St Andrews but could only manage 11th place.
The Englishman has been troubled by a long-standing calf problem and does not intend to play again before the WGC-HSBC Champions event, starting on 4 November.
Because of the machinations of the ranking system, which is calculated over a two-year period, he would knock Woods off the top spot on 31 October, provided the American sticks to his plan not to play until 4 November himself.
Westwood returned from a six-week injury lay-off last week to help Europe regain the Ryder Cup.
And although he had wanted to achieve the top spot on the course, the Worksop player stated it would be a dream come true to replace Woods, who has led the rankings since June 2005.
"I'll take it any way," said Westwood. "I've had a great year until getting injured. Look at all the world ranking points I've won - I was leading that by a mile before my injury.
"But I'm not allowing myself to think about it until it happens. It's something I've always dreamt of and it would be great if it happened."
The only way Woods can prolong his five-year reign would be to enter another tournament.
However, since he has already regained the number one spot from Ernie Els, Greg Norman, David Duval and Vijay Singh during his career, trying to wrestle it back off Westwood might prove an added spur on his return.
Westwood may have managed to play the Ryder Cup and the Dunhill Links Championship but he revealed he was some way short of fitness during both events.
"The calf got progressively more agitated and achy as the week progressed," explained Westwood after shooting a 73 in the final round at St Andrews.
"I haven't given it a chance to fully heal. I got to about 80% [fit] for the Ryder Cup, which I thought would be enough given the adrenalin that goes with it and all that.
"That was a tough week and this is not an easy week - six hours on my feet each day and only two days off between finishing the Ryder Cup and starting here.
"So it was never going to be right and it just got progressively more agitated and achy as the week's gone on - if it hadn't have been for [Westwood's businessman playing partner] Johann [Rupert] and the Dunhill I wouldn't have bothered coming.
"You won't see me back until I can practice fully and do myself justice."
If Westwood does not compete in Shanghai, he said his next tournament could be the season-ending Dubai World Championship, which begins on 25 November.