Thomas suffered a recurrence of an injury to his right hamstring while on European Cup duty with the Ospreys against Stade Francais last Friday and is currently undergoing medical assessment.
"We have begun investigations as to the extent of the injury, but as yet it's too early to say exactly what the repercussions are," said Wales team physio Mark Davies.
"We are monitoring the situation and there will be a scan this week. Due to the nature of the problem Adam has been called up to provide cover, but it is too early to speculate as to how long it will take Jonathan to recover."
Jones, 26, has been a prominent figure in the Scarlets' European campaign this season but has not been included in a Wales squad before.
"We've been impressed by his performances at regional level and over the last couple of months he has come on in leaps and bounds," added Wales coach Mike Ruddock.
"Having spoken to Gareth Jenkins about Adam, we are both of the same opinion that he has really started to fulfil his potential and is moving in the right direction.
"He had a particularly good game against Leinster in the Celtic league recently, competing well against a top lineout containing the likes of Malcolm O'Kelly.
"We feel we need cover in his position, with the injury to Jonathan and Ian Evans already ruled out through suspension and have every confidence that he has the ability to meet the challenge of international rugby if called upon."