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 spokesman, 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 US 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.

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.