Duncan had to be replaced in the 28th minute of Newcastle’s 2-1 semi-final win over Melbourne City on Saturday night after sustaining the injury.

Merrick confirmed on Monday morning that Duncan has sustained a grade-two quad strain and will miss Saturday’s decider against Melbourne Victory at McDonald Jones Stadium.

Duncan, 25, had started Newcastle’s past 12 matches and 19 all season, having usurped New Zealand international Glen Moss as the club’s first-choice keeper.

“Glen Moss will definitely be starting,” Merrick said.

“He’s just a terrific goalkeeper, he’s so experienced, he’s an international goalkeeper with New Zealand.

“These things happen in grand finals. Whenever we’ve lost a player, someone has stepped up and that’s happened again.”

Merrick said the former Australia youth international goalkeeper remained in good spirits despite the blow.

Newcastle regained Nigel Boogaard and Ronald Vargas following injury for Friday’s semi-final win.

Ben Kantarovski (hamstring) along with Duncan are on the injury list and Merrick was impressed with his side’s competition for spots.

“All the players are putting their hands up to play, whether they’re injured or not,” Merrick said.

“Ben Kantarovski is going to do some sprinting around today and he keeps doing exercises in front of me. The atmosphere is great. They’re buzzing.”