Former Celtic defender Kieran Tierney's injury that saw him miss Scotland's World Cup play-off has finally been explained.

The 25-year-old missed the crucial match against Ukraine at Hampden, along with a chunk of Arsenal's final Premier League matches.

At the time, details were scarce about exactly what was wrong with the Hoops academy graduate.

However, now the extent of the knock and how it was aggravated has been explained by Gunners club doctor Gary O'Driscoll in an episode of the All or Nothing documentary series.

"He had a hyperextension injury and finished the game, and he did not mention it, then played the next five games with no problems whatsoever," he said.

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"He then walked out of the shower on Thursday after training and turned a corner and bent his knee and felt the knee collapse."

Tierney was sorely missed by both Steve Clarke and Mikel Arteta in the final stretch of last season, with the national team being soundly beaten by Ukraine and Arsenal missing out on a top-four place along with Champions league football.

The Scot said: "I carried on playing for Arsenal, then reported for Scotland duty [in March] and played twice with no problems. I came back completely fine. Then I was doing some leg exercises in the gym and my left knee clicked.

“I instinctively felt it wasn't right and that I should probably go for a scan. I wasn't in any great pain. I'd played for Arsenal and we'd been winning, then for Scotland I got a goal and an assist. So there had been nothing to worry about after that Wolves match.

“But the scan result came back and suddenly I'm needing an op. It was so unfortunate. But after that click I just knew it wasn't normal. The verdict was 12 weeks, and it was gutting when I realised the games I'd miss for Arsenal and Scotland. However, there was nothing I could do about it.”