Duncan Weir has been ruled out of this year’s Six Nations, with the Glasgow Warriors stand-off to undergo surgery on his arm.
The 23-year-old, who gave Scotland a last-gasp victory over Italy last year, was injured during the 1872 Cup clash with Edinburgh at Scotstoun recently.
Subsequent examination has revealed he has severely damaged his bicep. The likely lay-off is three months.
Warriors head coach Gregor Townsend said: “I would like to think it won’t be the end of his season, but I think it rules out the Six Nations.
“With a bicep injury you are talking months rather than weeks.”
Weir’s absence would appear to make up Scotland coach Vern Cotter’s mind regarding the pivotal number 10 position.
Club-mate Finn Russell was preferred in the role during the Autumn Test series, and if he stays clear of injury is almost certain to start the Six Nations.
Stuart Hogg also faces a race against time, as the extent of his hamstring injury has yet to be determined.
Mike Cusack will make his first start for Warriors this season when they take on the Scarlets at Scotstoun Stadium tonight.
The Scottish qualified prop has battled injury and illness over the past two seasons, but his return is a big boost for the Warriors.
Following the defeat to Edinburgh Rugby at BT Murrayfield last week, Peter Murchie keeps his place at full-back and Tommy Seymour is retained on the left wing, while Sean Lamont comes in for Sean Maitland on the opposite side.
The Warriors have won their last 14 Guinness PRO12 fixtures at Scotstoun, and Townsend said: “We were disappointed with the performance and result last weekend and the players have been hurting since the game at BT Murrayfield.
“Tomorrow is a great opportunity for them to deliver a much improved performance and show our loyal supporters how hard they are willing to work to bounce back with a win.
“We face a tough challenge against the Scarlets, who are very well coached and have a number of quality players in their side.
“It’s great to be able to welcome back Mike Cusack for tomorrow’s match.
“He’s shown a lot of courage over the last couple of seasons and his hard work has been rewarded with a starting place in the team.”