LONDON - Rory McIlroy injured his left ankle while playing soccer, leaving the world's No. 1 player in doubt of defending his British Open title next week at St. Andrews.

In a shocking start to the week, McIlroy posted a photo on Instagram showing him on crutches with a walking boot on his left ankle. He said it was a "total rupture" of an ankle ligament and the joint capsule from kicking around a soccer ball with friends in Northern Ireland.

"Continuing to assess extent of injury and treatment plan day by day," McIlroy said on the post. "Rehab already started.... Working hard to get back as soon as I can."

His spokeman, Sean O'Flaherty, said he was definitely out of the Scottish Open, which starts Thursday at Gullane. O'Flaherty said they would not know until the end of the week the prospects of McIlroy teeing it up at St. Andrews on July 16.

McIlroy was the joint favorite at St. Andrews with Jordan Spieth, the 21-year-old Texan who has won the Masters and U.S. Open and goes to the British Open as only the fourth player with a shot at the modern Grand Slam.

McIlroy referred to his ATFL, which is the anterior talofibular ligament and the one most commonly sprained. The left ankle is crucial in a golf swing as weight shifts to that side while generating power.

"That's a big blow to the Open if he misses it," former Ryder Cup captain Sam Torrance said from Wimbledon.

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McIlroy won the Open last year at Royal Liverpool by going wire-to-wire and taking a six-shot lead into the final round. He also won the PGA Championship, joining Jack Nicklaus, Tiger Woods and Bobby Jones as the only players in the last century with four majors at age 25 or younger.

Still to be determined is whether McIlroy can do the same as Woods, who played through a stress fracture and torn knee ligaments when he won the 2008 U.S. Open in a playoff that lasted 19 holes.

Woods was determined to play in that major because it was at Torrey Pines, one of his favorite courses on tour.

McIlroy already has a reasonable connection with the Old Course, where he secured his European Tour card in 2007 as an 18-year-old at the Dunhill Links with a third-place finish. He also tied the major championship record of 63 when the British Open was held at St. Andrews in 2010, though McIlroy followed with an 80 in the wind. He tied for third at St. Andrews five years ago.