He woke up with intense pain in his tendons and joints and despite getting through the event with stretching and anti-inflammatories, the problem worsened.
Mickelson started taking his medication around the time of the USPGA Championship in August and said it took a full eight weeks for it to kick in.
That would have been a week after the Ryder Cup, where he defeated Peter Hanson in the singles.
"I feel terrific," said the left-hander. "I've been able to do all the normal activities in my life. I've been able to practice at whatever level, and I honestly don't feel any different than I did before."
Mickelson's attention now shifts to Shanghai, where he is the defending champion and has been paired with England's Lee Westwood and former USPGA champion YE Yang for the opening round.
Woods & Westwood ready for China test
On Sunday, Westwood ended Tiger Woods's 281-week reign as world number one but the mantle could again switch hands at the Sheshan International Golf Course.
Westwood has a narrow lead over Woods and PGA champion Martin Kaymer, with Mickelson in fourth.
"It's one thing getting there, another thing staying there. It's obviously very close at the moment," said Westwood.
The Englishman has been suffering from a calf injury suffered in July and has played only one tournament since the Ryder Cup.
"It (the injury) is still not right," he added. "But I was getting bored of sitting at home doing nothing and all of the people I've seen have said I can't do any more damage to it, so to come out and the walking around may do it a little bit of good.
"It's going to be tough this week. I have to play well, which may not be easy because I'm bound to be a bit rusty because I've not played a lot just recently."
Mickelson is likely to need at least a fourth-place finish to be in with a chance of clinching top spot, depending on how his rivals fare.
When asked if it was frustrating to see the number one ranking fall into someone else's hands after all the time he spent at number two behind Woods, Mickelson had no complaints.
"I haven't played well since the Masters, or as well as I expected maybe since the US Open," he said.
"So I don't feel like I've earned it. I've got to play well and win more tournaments and then I feel like those results will take care of itself."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.