Thomas was forced to sit out Saturday's 1-1 draw at Carlisle with a knock and is still struggling.
The player has been filling in at right-back as Graeme Murty is still sidelined with a knee injury, so midfielder Lloyd James will now be drafted into the back four again.
Young winger Kayne McLaggon is the only other injury absentee with his Achilles problem.
Back-up goalkeeper Bartosz Bialkowski has joined Barnsley on an emergency loan so Tommy Forecast will be promoted to the bench as cover for Kelvin Davis.
Rickie Lambert will line up in attack against the side for whom he
scored 59 goals in three years before joining Saints earlier this season for £1million.