Hoolahan, the former Republic of Ireland international and Norwich City club legend who signed with Newcastle six weeks ago, suffered an ankle injury in Tuesday's 1-0 loss to Adelaide in the FFA Cup quarter-finals.

The 37-year-old is understood to have suffered a syndesmosis, or high-ankle sprain, but will have to wait until Monday to visit a Sydney-based specialist for a full prognosis. He is facing a recovery period of about 12 weeks, which would rule him out for the first two months of the A-League season.

Dubbed "the Irish Messi" by Norwich fans, Hoolahan had been forging a promising partnership with Panamanian striker Abdiel Arroyo.

In his absence, Merrick said Newcastle would probably opt for a three-man strike force, instead of playing with two No.10s, as they have so far this pre-season.

"He's a bit special as a player, especially as a creative player," Merrick said.

"I'm not sure that we've got anyone that creative, but we've certainly got goalscorers up front that can play that role. It just means a slight change to the system that we're playing."

Merrick said there were "a number of players" Newcastle could slot into their forward line, including Angus Thurgate, Jason Hoffman and Kaine Sheppard, starting with Saturday's trial match against Sydney in Dubbo.

The coach said Hoolahan had suffered "a complicated injury" but did not want to speculate on the possibility that he might require surgery.

Adding to Merrick's frustration, Hoolahan conceded a free kick for the incident that left him injured.

"The first rule of any referee is to make the game safe for the players to participate," Merrick said.

"I'm not sure that was the case on Tuesday night."

Merrick described the tackle as "awkward" but was willing to give the Adelaide player involved, Ryan Kitto, the benefit of the doubt.

"I don't think there was any malicious nature to the tackle," he said.

"But there were a few other tackles that I thought were a bit disappointing."

The overall performance of the match officials left a lot to be desired, Merrick said.

He disputed the decision to send off Lewis Italiano, which left the Jets with 10 men for 80 minutes and means the goalkeeper will be suspended for Newcastle's first game of the A-League season, against Central Coast in Gosford on October 19.

And he also felt that Adelaide skipper Michael Jakobsen produced a "blatant" hand-ball that went unpenalised.

"Two difficult decisions made life very hard for our players, who I give so much credit to, because they fought for 80 minutes, with 10 men, and battled and only lost by one goal," Merrick said.

Jets skipper Nigel Boogaard is set to return from a knee injury against Sydney, while Irish import Bobby Burns will make his debut.