Duncan triggered a release clause, believed to be about $350,000, on Monday and is expected to join Saudi Arabian club Al-Qadsiah, who recently signed Victory defender Rhys Williams.

The Duncan deal was done in less than two-days and came out of left field.

Duncan is the second player to be released by the Jets in the past week, following the return to England by midfielder Wayne Brown.

Jets coach Ernie Merrick, despite the club receiving a significant transfer fee, was unhappy at losing Duncan.

“I’m extremely disappointed to lose Jack, but he’s been offered an opportunity that he wants to pursue,” Merrick said in a statement. “At his age and with his ability, I think Jack has the potential to go onto big things and I wish him the very best with the next step in his career.”

Goalkeeper coach, Chris Bowling deputised for Glen Moss in the Jets’ 2-1 win over Shabab Al-Ahli early Tuesday morning in Murcia, Spain.

Jets chief executive Lawrie McKinna and youth team keeper Noah James travelled to Spain on Tuesday from China where the under-18s have been competing at the Weifang Cup.

McKinna and Merrick will discuss options for a second keeper.

The Jets are fortunate to have a proven performer in former New Zealand international Moss on the roster.

Moss, 35, was outstanding in 11 appearances last season, including the 1-0 loss to Victory in the grand final.

The Jets had three keepers on the books for the first half of the campaign, after veteran back-up Ivan Necevski signed a short-term deal to cover for Moss’ international commitments and a foot injury to Duncan. He turns 39 midway through next season. There are other keepers in the NSW and Victorian national premier leagues with A-League experience.

James has represented Australian under-20 but is only 17.

Duncan returned to the Jets in 2015 for a second stint after a season in Denmark at Randers, where he didn’t play for the first team.

The 25-year-old played 19 appearances for the Jets last season but missed the 1-0 grand final loss to Melbourne Victory due to a torn quadriceps.

Duncan said although he loved his time in Newcastle, the time was right to move.

“I made my A-League debut with the Jets, the club has given me so much,” Duncan said. “This is a great opportunity to develop my career and one that I couldn’t pass up.”

The Jets came back  from a goal down against Shabab Al-Ahli. Jason Hoffman equalised in the 40th minute after collecting a defence-splitting pass from Steve Ugarkovic.

Kosta Petratos scored the winner off the bench in the 65th minute.

The Jets play Saudi club Al-Fayha Wednesday night.