BBC Scotland's chief sports writer
Scotland captain Sione Tuipulotu has been ruled out of the Six Nations after he picked up a pectoral muscle injury while training with Glasgow Warriors.
The 27-year-centre, who will undergo surgery, did not feature in Glasgow's weekend Champions Cup loss to Harlequins, though he had been rested for the match anyway.
The upper body injury also casts doubt on Tuipulotu's chances of touring Australia with the British and Irish Lions this summer, though he is expected to return before the end of the domestic season.
Flanker Rory Darge, 24 and fly-half Finn Russell, 32, will captain Scotland in Tuipulotu's absence.
Capped 30 times since 2021, Tuipulotu has scored three Scotland tries.
The Scots are also likely to be without lock Scott Cummings, 28, for the whole of the Six Nations after he came off against Harlequins with an arm fracture. Cummings' injury will be further assessed.
Gregor Townsend's side open the championship at home to Italy on 1 February.
Townsend will not be call up additional players to his squad as things stand.
'One of the form inside-centres in the world'
Sione Tuipulotu is one of the form inside-centres in the world game; a captain, a physical brute, an artist, an inspiration to his team. If he's missing some, or all, of the Six Nations then a chunk of optimism about Scotland's campaign is going to go with him.
His partnership with Huw Jones is a cosmic blend. Now it looks like an alternative is needed.
Jones and Tom Jordan does not pack enough physical punch. Franco Smith never plays them together at 12-13 for Glasgow.
Most likely, it is going to be another Warriors pair, Stafford McDowall and Jones, in the midfield in the championship. That is still a forceful and dynamic unit.
As for Tuipulotu's fitness for the Lions, let us just presume that whatever time he misses in the Six Nations he will still have time enough to get himself roaring again for the summer in Australia.
'Absolute disaster' - Your views on Tuipulotu injury
John: This will be an absolute disaster for Scotland. First and foremost, I hope that he recovers quickly. He is an exemplary leader and player.
Ben: The injury to Tuipulotu plus Glasgow's result against Harlequins has definitely put a dampener on optimism for the Six Nations. If we want to be in with a serious chance at the tournament, we need all of our big players available and playing consistently at eight or nine out of 10's. Zander Fagerson, Finn Russell & Tuipulotu we need the most.
Henry: Tuipulotu's injury was obvious last week and he should have been withdrawn immediately. Now he and George Horne are doubtful Scotland starts. No Warriors excuses. We did it the hard way last year and can repeat that this time.
Bill: Of course the loss of a world class player is a blow to a squad which is aiming to bring on a number of newer talents supported by several established stars, but we must stop focusing on negatives and reorganise around our other game changers including, in particular, Huw Jones and Blair Kinghorn.