Medics believe he will not be available for selection for a minimum of three months.
Head Coach Nigel Davies said: "It is very disappointing and unlucky for Mark at this particular time. It's one of the curses of modern day rugby unfortunately - it is a hugely physical sport and these boys train intensively on a daily basis.
"However, Mark is one of the most positive players I know in terms of coming back from injury. He deals with these set backs more positively than most people. He's been a huge asset for already for us this season in his role as captain, is totally respected by all the players and leads by example.
"He's a true Scarlet and embodies all the values that we belong to as a rugby region, respects our heritage and demonstrates his commitment to the Scarlets on and off the field.
"Mark will be back with us in a week or so for his physio treatment. Despite the injury, I know he will continue to play a key leadership and motivational role off the field with the players, as well as working closely with coaching staff and Scarlets management."
