The Llanelli-based region are currently two places below the play-off spots, but are only five points behind fourth placed Leinster and will be looking to take points off current league leaders Munster when the sides meet at Parc y Scarlets on Saturday.
"Jon's looking very doubtful, his dead leg is taking a long time to heal and it could be one to four weeks out," said Scarlets head coach Nigel Davies.
"To have Jon in the mix would be great but we have other good centres and Sean Lamont can also play there."
Davies has been the in-form Welsh centre this season and was ever-present during the Dragons' Six Nations campaign.
But the news will be softened by the return of fit-again international trio George North, Stephen Jones and Sean Lamont to face Tony McGahan's men.
North has been sidelined with concussion since the league clash with Benetton Treviso, Lamont with a hamstring strain which has kept him out of action since the RBS 6 Nations, while Jones looks to have recovered from the wrist injury that forced him off against Ulster.
"Stephen should be okay and George and Sean are also both fine," added Davies.
Munster need just one more win to secure top spot in the Magners League while just two points will see them earn a definite home semi-final.
"Munster are such a quality side, they're used to winning and have that mentality bred into them.
"They're top and will want to come here and continue their fine league form. I wouldn't be surprised if they come with their first choice side.
"It is simple for us we have to win our three games to have any chance of qualifying. We have to be up for the challenge."