Former Republic of Ireland international Hoolahan sustained ligament damage to his right ankle during last week's FFA Cup quarter-final defeat to Adelaide.

The 37-year-old underwent surgery on Monday and the Jets announced Hoolahan will miss the first half of the A-League season with a likely return in January.

Merrick said the club would be actively seeking a replacement while he is sidelined.

"We've got space in the squad obviously and we've got the finance I think, so we'll certainly be looking at is there someone out there that can fit the bill," Merrick said.

"Because it's this time of year there's less money in it. You're not paying them for 12 months.

"Sometimes someone is disillusioned with where he's at or comes back from overseas or is not getting a game with an elite club, so sometimes they just pop up."

In the short-term Hoolahan's injury will force Merrick to alter his tactics for the Newcastle's A-League opener at Central Coast on October 19.

Having played the ex-Premier League midfielder in a dual No.10 role with Dimi Petratos during the pre-season, Merrick said he'll be reverting back to a more conventional shape without Hoolahan.

"He was virtually a second No.10, so we'll go back to a traditional wide player in our team," Merrick said.

"We'll go back to a more traditional three which got us into a grand final in year one."

Australian Associated Press