Phil Mickelson's title defence at this week's Waste Management Phoenix Open remains in doubt following the British Open champion's withdrawal from the Farmers Insurance Open because of a back injury.

Mickelson made the halfway cut at Torrey Pines but was then forced to pull out of the event before the start of Saturday’s third round.

The 43-year-old, who finished joint second in Abu Dhabi on the European Tour the previous week, then flew to Georgia for a consultation with back specialist Tom Boers.

“My facet joints had locked up,” Mickelson said in a press release. “Tom restored my mobility, but I still have inflammation that will take a week or two to subside.”

Mickelson, who has opted out of yesterday’s pre-tournament pro-am, added: “If it was any other tournament I’d skip it, but I’m defending, it’s my second hometown and I love the event. I’ll have a light practice session and if it goes well I may try to play.”

Mickelson won by four shots last year, leading from start to finish after a stunning opening round of 60 , after the left-hander’s birdie putt on the last hole lipped out.

If Mickelson is unable to play, English duo Ian Poulter and Lee Westwood will be among those hoping to take advantage of the five-time major winner's absence at TPC Scottsdale.

Westwood is making his debut in the event, which is famous for its raucous crowds, after opting not to compete in the Desert Swing on the European Tour following his move to the United States at the end of 2012.

The former world number one has slipped to 26th in the rankings after failing to win a tournament in 2013, his best finish being a share of third in the Open at Muirfield where he led by two shots going into the final round.

Westwood and Poulter finished joint 47th at Torrey Pines last week.