"Jonathan Davies has done his pec muscle, which will keep him out of the rest of the autumn campaign," said Gatland.
"It's a huge blow for Jonathan. He was a big loss to us in the first half. He'd started to make a couple of breaks and he was strong on his feet. He's a pretty important player for us at the moment and he gives us that left footed option, too.
"He's pretty disappointed. The physios have said it might settle and he could be back in December, but if he needs an operation that could be five months out and he could be out of the Six Nations as well."
The 25-year-old has been in sensational form throughout 2013, starring for Wales in the defence of their RBS 6 Nations crown and guiding the Scarlets to the semi-finals of the RaboDirect PRO12.
He went on to start all three Tests on the British & Irish Lions tour of Hong Kong and Australia as the Lions headed home with a first series victory since 1997.
Davies has started both games in this season's Heineken Cup, with the Scarlets shocking Harlequins in Round One before drawing with Racing Metro in Round 2. If he does require an operation, he will miss the four remaining matches in the group stages as the Scarlets go in search of a first quarter-final spot in seven seasons.
The Scarlets currently top Pool 4 with six points ahead of next month's double header with last season's beaten Heineken Cup finalists ASM Clermont Auvergne. Simon Easterby's troops travel to the Stade Marcel Michelin on Saturday, December 7 before hosting Wesley Fofana and co at the Parc-y-Scarlets the following week.