The problem is described as wear and tear and is a legacy of a long season for Toulouse in France followed by the British Lions tour to New Zealand in the summer.
Thomas captained the Lions in the last two Test matches after Brian O'Driscoll was ruled out early in the opening encounter.
The 31-year-old Thomas is considered something of a talisman in the Wales camp, so his absence from the All Black encounter could prove a real setback.
Not only that, but his name has now been added to an increasing list of injuries coach Mike Ruddock has had to endure in the opening weeks of the season.
Another France-based Lion - Clermont Auvergne outside-half Stephen Jones - is a doubt to face New Zealand because of an ankle problem.
Lions prop Gethin Jenkins is already sidelined for all four games against New Zealand, Fiji, South Africa and Australia, while Gavin Henson, who toured this summer, is still a worry following groin surgery.
There is better news for Ruddock regarding another Lions trio as number eight Ryan Jones, centre Tom Shanklin and scrum-half Gareth Cooper who are all on the comeback trail.
Also in the frame for the autumn series is Lions wing Shane Williams who is targeting a return to Ospreys colours against Connacht on Friday week.
Ruddock's long-term resources have also been boosted by the return to action this weekend of wing Mark Jones after being plagued by knee problems over the past two seasons.