Rangers have confirmed that Steven Naismith will miss the rest of the season.
The in-form forward was stretchered off in Saturday's 2-1 win at Aberdeen following a clumsy challenge on midfielder Rob Milson.
Scans have confirmed that the 25-year-old suffered damage to the anterior cruciate ligament in his right knee.
"This is devastating news for Steven and our thoughts are with him," manager Ally McCoist told the club's website.
"He was in the form of his life and it is a huge blow for him and the club that he is out for the rest of the season.
"He will get the best possible treatment in the weeks and months ahead and everyone at Rangers Football club will support him during his rehabilitation period."
Naismith suffered a ligament problem in his left knee in 2008 that ruled him out for eight months and the club confirmed he will be treated by the same surgeon who operated on that occasion.
"Naisy has recovered from a serious knee injury before and will come back again," McCoist added.
"He is a fighter and a popular member of the squad and we will all work hard to win games for him while he is out."
Naismith is the Scottish Premier League's leading scorer with nine goals in 11 matches and has become a fixture in Craig Levein's Scotland side.
And the international manager was quick to assure Naismith that he will be back in his plans as soon as he is fit again.
"Having had to stop playing myself with a similar injury, I know just how frustrating it is and how soul destroying it can be to suffer from this particular injury," said Levein.
"The good thing is the attitude of Steven will be what enables him to get back, not just playing well, but to that gradual curve of improvement he was on already when he came back from a similar injury previously. His progress has been quite phenomenal.
"Knowing the type of boy he is and his attitude, I spoke with him the other day and he said all the right things that will help him to recover, I'm certain he will be back probably for pre-season training and he will very quickly get back on that curve of improvement.
"As soon as Steven Naismith is available and fit, he will be back in the squad."