The Scarlets centre has been out of action since February but had hoped to make a return to action before the end of the season.
The Wales fly-half has committed to the club, ending speculation he was set to become the latest Wales international to leave the country.
Wales winger can help take Northampton to the next level, says Jim Mallinder.
The Scarlets have withstood a late Connacht fightback to get a bonus point win in the Pro12 this afternoon. Tries from Jonathan Davies and Josh Turnbull, and two touchdowns from Gareth Davies and helped seal the victory.
Jonathan Davies's try gave the Scarlets the lead at home against Connacht two minutes into the game. A conversion and two penalties from Rhys Priestland leaves the Scarlets 13-6 ahead at half time.
Wales fly-half Rhys Priestland has extended his contract with the Scarlets, putting an end to recent speculation over his rugby future.
The 26-year-old had been linked with clubs in England and France, as well as the possibility of a contract with the Welsh Rugby Union.
This evening's Pro 12 match between Scarlets and Ulster at Ravenhill has been postponed after referee Leighton Hodges deemed the pitch unplayable.
The two teams are now exploring the possibility of playing the game tomorrow at Ravenhill, with tickets purchased for tonight's game valid for the rescheduled match.
Tries from Jordan Williams, Kristian Phillips and Gareth Davies, as well as two from Frazier Climo, help the Scarlets secure victory over Treviso.
It's been a good night for the Scarlets. Despite being 4 points behind at half time, a try from hooker Phillips and 14 points from man of the match Rhys Priestland gave them a huge win.
Meanwhile there are reports that the fly-half will stay with the region despite being linked with French and English clubs.
Hosts Racing Metro are four points ahead of the visitors at half time. The Scarlets are in Paris for the fifth round of the Heineken Cup.