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.
Hosts Scarlets lost against the Ospreys in the Pro12 derby this afternoon.
Despite going into the second half 3-0 down, the Ospreys put on an impressive defensive at Parc y Scarlets to secure their win.
The boot of Rhys Priestland put the 6 point score on the board for the Scarlets with Ospreys' Dan Biggar also scoring one of his own.
The only try of the game was from Ian Evans who touched down five minutes from time.
Final score Scarlets 6 - 10 Ospreys.
The Scarlets have suffered a heavy defeat away at Clermont in the Heineken Cup. They were beaten 32 points to 11 by the French giants. Simon Easterby's men will now welcome today's winners to Parc y Scarlets for the return fixture next weekend in Round 4 of the competition.
The Scarlets have just kicked off in their Heineken Cup tie at Clermont. Simon Easterby's men will be looking to extend their lead at the top of Pool 4 with a tough win in France.
One of the Scarlets' most high-profile players is returning to the club next season, moving from Heineken Cup finalists Clermont Auvergne.
Regan King, 33, has signed a new two-year deal with the region he calls his 'spiritual rugby home'.
Scarlets Head Coach Simon Easterby said: "To have the quality and experience of a player like Regan King back in our squad next season will be a big positive for our region as a whole.
"Not only will we have secured a rugby player of real quality on the pitch who has played at the top end of European and French rugby for the past two years, but we've also gained the value of a professional who has such rounded experience which can be passed onto others.
"He's shown his inherent quality as a player in his recent performances in the Heineken Cup as well as the domestic French game and is a player any club in Europe would want in their ranks."
New Zealand-born King achieved his All Blacks cap against Wales at the Millennium Stadium in November 2002. He also played for Paris based club Stade Francais before leaving to join the Scarlets in the 2005/6 season.
He has appeared 143 times in a Scarlets shirt with 180 points.
King's last game for the Scarlets was against Cardiff Blues at home in front of a packed 11,000+ Parc y Scarlets crowd on May 6th 2011.
The Scarlets have beaten Newport Gwent Dragons in the first round of the LV Anglo-Welsh cup.
A late penalty from Rhys Priestland secured the Scarlets a 16-16 draw at Zebre. It meant the home side were denied a first ever home win in the RaboDirect Pro 12.
A 77th minute penalty from Luciano Orquera gave Zebre a 16-13 lead, before Priestland hit back in the last action of the game.
Scarlets head coach Simon Easterby said he was "delighted" after the Scarlets won away at Harlequins in the Heineken Cup - but they now need to follow this victory with another next weekend.
Rhodri, Scott and Jordan Williams all scored brilliant tries, and Rhys Priestland kicked 18 points, on the way to a 33-26 win at the Twickenham Stoop.
"I'm delighted with the way we went about our business," Easterby said.
"It's vital that you win your home games in the Heineken Cup and if you can get points on the road then that's a big advantage."
"But this won't mean anything we don't get a result at home nest weekend. This was a big performance but we have to keep our feet on the ground."
Racing Metro - now featuring Dan Lydiate and Jamie Roberts - travel to the Parc y Scarlets next Saturday.
Fellow French heavyweights Clermont Auvergne - who were runners up in last season's Heineken Cup - are the final side in Pool 4.
The Ospreys were beaten by three-time champions Leinster, in their first Heineken Cup game of the season.
Leinster won 19-9 at the Liberty Stadium, with Ireland flanker Sean O'Brien scoring the game's only try.
Dan Biggar kicked three penalties for the hosts.
The Welsh side had a great opportunity for a try after 24 minutes, but Richard Hibbard was tackled, after deciding to go for the line himself, with teammates outside him.
Leinster won a penalty at the scrum, counter-attacked, and four minutes later they were ahead, having shown a clinical edge the Ospreys lacked.
Sean Cronin broke through, and O'Brien touched down.
Leinster were firmly on top at the breakdown in the second half, and although the Welsh side looked dangerous in attack, they wasted more chances.
Fly-half Jimmy Gopperth kicked a penalty with less than two minutes remaining, to deny the Ospreys a losing bonus point.
Ospreys backs coach Gruff Rees said: "At key moments we made bad decisions. We were not smart under pressure, and they were the opposite. They did the right thing at the right time."
"We have got to get a smartness and urgency into our game because at times we had no emotional control, which was disappointing."
The Scarlets have got their Heineken Cup campaign off to a thrilling start, with three stunning tries to beat English Premiership side Harlequins at the Twickenham Stoop.
The Llanelli side showed their attacking potential with a first score within eight minutes. Young scrum half Rhodri Williams touched down, after slick passing on the right touchline.
Harlequins pegged them back with a try, conversion and penalty, but the Scarlets extended their lead to ten points at the break.
Wales centre Scott Williams showed his power and pace, breaking the line before speeding away from the covering defenders.
The Scarlets 20-year-old winger Jordan Williams then bounced off two tackles, outpaced a third, and stepped inside a fourth, to make it 33-19, with 20 minutes remaining.
Harlequins got back into again with a try of their own, but the Welsh side held on for a 33-26 victory.
Tries for Rhodri Williams and Scott Williams and eight points from Rhys Priestland's boot put the Scarlets ahead at half-time away to Harlequins in the Heineken Cup. Harlequins 10-20 Scarlets (HT).