Glasgow Warriors co-captain Callum Gibbins is out of the European Champions Cup tie with Saracens on Saturday.
The back-row forward was injured in the first ten minutes of Glasgow’s thrilling 33-24 win over Cardiff Blues at Scotstoun on Sunday.
Warriors have called flanker Matt Smith into the squad as “additional player” to compensate for Gibbins’ absence.
Clubs are allowed a maximum of 41 players in their Champions Cup squads and Glasgow have had to de-register Gibbins to make space for Smith.
Glasgow coach Dave Rennie said after the match that Gibbins’ injury “doesn’t sound good”, adding “he might have injured three things”.
Gibbins appeared to hurt his knee, while fellow back-row forward Matt Fagerson was forced off with a shoulder injury.
The bonus-point win over Cardiff edged Warriors closer to a quarter-final place and they trail Pool 5 leaders Saracens by four points going into Saturday’s game.
But Glasgow’s place in the last eight could be sealed on Friday night if Edinburgh beat Montpellier in Pool 3.