Roberts-Thomson injured his knee in the reigning premiers' clash with Geelong on April 19 and the early prognosis was positive after an MRI scan.
Click here to see the full round six injury list
But an arthroscope revealed more joint surface cartilage damage in his right knee, with Swans physiotherapist Matt Cameron suggesting he could need two months to recover.
"We need to tidy that up a little bit, which means he will be out for a little bit longer," Cameron told the club's website.
"It's an injury that can really be a little bit unpredictable and one we have to be a little bit more careful with, so it could even push out to eight weeks with him out of the game.
"It is a bit disappointing, but it's a bit unpredictable and if things go well we might be able to get an early return."
Roberts-Thomson's absence has further stretched a Swans defence which lost Alex Johnson to a season-ending knee injury in March.
Veteran rebounding defender Rhyce Shaw is expected to return in Sunday's SCG clash with Brisbane, but Cameron noted it was no fait accompli.
"He's running well at the moment and he ran really fast yesterday and he's due to start training with the team on Wednesday," he said on Tuesday.
"It's now been a three-week injury so we won't be taking a chance if we feel he's not quite right or hasn't had quite enough preparation to put himself out there under the rigours of playing footy."
Meanwhile, Gary Rohan is slated to return to team training next week after spending more than a year on the sidelines due to a broken leg.
"Gary is running really well and has done lots of speed and agility work and he's ready to start training with the team," Cameron said.
"He's going to do some individual sessions this week with the footy and we'll have him in with the team next week."
