"Scans of the injury have been done but it is still too early to fully diagnose the extent and severity of his injury at this stage, and whether he will definitely need surgery.
"From the initial reports we have had, I would not expect him to be available for at least the next 12 weeks."
Scarlets Head Coach Nigel Davies said: "Obviously this is another disappointment for the Scarlets on the injury front in recent weeks and is very unfortunate for Matthew who has been playing with distinction this season following a great Lions Tour in the summer.
"The injury will rule him out for a while but we have belief in the strength and depth of the squad this season.
"As I have said already, it is frustrating for us to have key players side-lined, but it¹s rugby - we will pick up injuries and we have to be realistic and pragmatic about that. We are in a much better position than we were last season in terms of the choices we have available to us across the squad.
"We¹ve got to remain focused on the job in hand which is to prepare well for some very big games home and away in December and back the players that we do have available to us. They¹re a very committed and eager group and will relish the opportunity to step-up and prove their worth on the field.
"It's important that we all give the players in the Scarlets squad our complete backing and belief during a crucial month for the region."