Finn Russell is a doubt for Scotland's Six Nations clash with France on Saturday after he suffered a head injury playing for club side Racing 92.
The fly-half failed an initial Head Injury Assessment (HIA) after being replaced in the Top 14 match against Toulouse this past weekend, but has nonetheless joined up with the Scotland squad.
He has been declared symptom-free but faces a race against time to prove his fitness, with World Rugby protocols demanding he undergoes at least six days of further tests.
That would mean he could only be declared fit on match-day, so Scotland have called up six players as cover with fly-half Duncan Weir among them. Meanwhile fullback Stuart Hogg will definitely miss the trip to Paris.
Russell is the latest in a string of injury problems that have beset Scotland's campaign and left head coach Gregor Townsend short of options throughout the pitch.
He is now subject to return to play protocols which mean he will have to show no symptoms of concussion throughout the week to be considered.
Hogg and centre Huw Jones both suffered injuries in the 22-13 defeat to Ireland at Murrayfield earlier this month. Jones sustained knee ligament damage which will very likely rule him out of the remainder of the tournament while Hogg left the pitch with a shoulder injury after a heavy collision with Ireland's Peter O' Mahoney.
Townsend has moved to strengthen his squad with the addition of Worcester Warriors' in form fly-half Duncan Weir. The stand-off has been in exile from the national side for two years but has impressed at Worcester, scoring 169 points this season.
Elsewhere Edinburgh trio Magnus Bradley (back-row), Dougie Fife (wing) and James Johnstone (centre), and Glasgow Warriors pair Zander Ferguson (prop) and George Turner (hooker) have also been brought into the squad.
They replace the injured George Horne and Stafford McDowall (both of Glasgow Warriors), David Cherry and Murray McCallum (Edinburgh) and Jake Kerr (Leicester Tigers).