Scotland prop Allan Jacobsen faces four months on the sidelines after tests showed he will require shoulder surgery.
Jacobsen damaged his shoulder during Scotland's 26-6 RBS 6 Nations victory over Italy on Saturday.
Scotland team doctor James Robson said: "Chunk has damaged the pectoralis major muscle adjacent to his right shoulder.
"It will require repair and the operation will be carried out later this week by surgeon Richard Nutton at the Spire Murrayfield Hospital."
The loss of the Edinburgh forward for the remainder of the Six Nations is a major blow to head coach Frank Hadden.
Jacobsen, 30, played in all 10 of Scotland's internationals in 2008 and has also been ever-present in the tournament so far.
Hadden said: "Chunk has been in a rich vein of form and we wish him a speedy recovery."
Hadden received better news on lock Alastair Kellock and centre Max Evans.
Kellock, who was recalled in the absence of the injured Nathan Hines and Jim Hamilton, strained ankle ligaments during the Murrayfield victory but he will be fit for the visit of Ireland on March 14.
Dr Robson said: "He is now wearing an immobilising boot and will begin his rehabilitation under the care of the Glasgow Warriors physiotherapy staff tomorrow with a view to being able to resume training in around seven days."
There were fears Glasgow team-mate Evans had suffered a fractured cheekbone but X-ray results allayed those.
"Max has severe bruising to his right cheek but, happily, the X-ray has indicated there is no break," added Dr Robson.
England Under-20s head coach Martin Haag was full of praise for his World Rugby Under-20 Championship-winning side after Harry Mallinder inspired them to a 45-21 win over their Irish counterparts and a third title in four years.