Scotland captain Sione Tuipulotu has been ruled out of the Six Nations after injuring his pectoral muscle during a training session with Glasgow Warriors last week Scottish Rugby said on Monday.
Scott Cummings' participation is in doubt after he broke his arm at the weekend.
Tuipulotu took over as captain ahead of last year's November internationals.
Rory Darge and Finn Russell have been named as co-captains for Scotland's Six Nations campaign, which starts at Murrayfield against Italy on Feb. 1.
The injury also casts doubt on Tuipulotu's hopes of touring Australia, where he was born, with the British & Irish Lions in the summer.
"Sione will undergo surgery this week and is expected to return to action before the end of the season," Scottish Rugby said.
The pair led the side in four matches in the 2024 championship, while Russell was captain when they started the campaign with an away win over Wales in Cardiff.
Cummings sustained an arm fracture early on in Glasgow Warriors' defeat by Harlequins in the European Champions Cup on Saturday.
"He is also likely to miss the tournament and will undergo further review to determine when he will be fit to return," Scottish Rugby said.
Scotland coach Gregor Townsend, who named a 37-man squad for the tournament last week, plans no additions to the squad at this point.
