Hoolahan, the former Ireland international and Norwich City playmaker, was carried from the field early in the second half after a clash with Adelaide's Ryan Kitto.

The 37-year-old will undergo scans on Thursday but Jets coach Ernie Merrick said the early indications were that he had suffered a syndesmosis, or high-ankle sprain.

"He got kicked from behind and then he twisted his ankle forward, and then he got a free kick against him ... it looks like he's going to be out for a while," Merrick said.

Merrick said it was likely to be "a minimum three-month injury".

"We'll obviously have it scanned, have it confirmed, the extent of the damage," he said.

"But the two previous similar-type injuries I've seen are a minimum of 12, 13, 14 weeks. Minimum. Sometimes they have to operate to put a screw in to hold the two bones together, because the ligaments and tendons are all ruptured to death. So it's been a very costly experience coming here."

Newcastle's first game in the A-League is their round-two showdown with Central Coast at Gosford on October 19.

Hoolahan, a club legend at Norwich after playing for a decade with the Canaries, also appeared in 43 internationals for Republic of Ireland.

Although in the twilight of his career, signing a player of his class and experience was considered a coup for the Jets, who will now be sweating on his recovery.

Merrick was disappointed with the refereeing in Newcastle's 1-0 loss, after goalkeeper Lewis Italiano was controversially sent off and Adelaide's Michael Jakobsen avoided conceding a penalty for what Merrick said was a "blatant" hand-ball.

"We have still got this system where the referees don't seem to refer to the linesmen and the linesmen don't say anything," Merrick said, adding that the VAR is not used in the FFA Cup.

He was also disappointed that Hoolahan was penalised for the incident in which he was injured.

"It's hard to complain when you lose a game," he said.

"But these things are very tough to take because I have been in this league a very long time and the refereeing hasn't improved.

"And you [media] are probably going to get me fined because I know you're going to make the most of what I have just said."

Hoolahan's injury is a setback because his combination with Panamanian striker Abdiel Arroyo and Socceroo Dimi Petratos had been improving with each game.

Now, if Merrick's worst fears are realised, his most experienced player is likely to miss the first quarter of the season.

AAP reports: Merrick has warned that allowing more foreign players into the A-League is "ridiculous" and will threaten the survival of some clubs.

Merrick says the gap will increase between cashed-up clubs and financial battlers if the number of visa players rises.

A-League clubs are allowed a maximum of five visa players but there's a push to increase that number.

Merrick says the number of foreigners should be reduced.

"I would prefer that we're down to four to give more Australian players an opportunity," Merrick said.

"But not five, six, they're talking about going up to seven I think - that would be ridiculous for us, the development of Australian football ...

"We would be bringing in pretty low-class players. We would be better developing Australian players."

Merrick cited the scenario in his native country Scotland as evidence of the damage done by permitting too many foreign players.

Newcastle on Wednesday announced Northern Ireland under-21 international Bobby Burns would join the club on a season-long loan deal from Scottish Premier League club Heart of Midlothian.

"Hearts have got 30 senior professional players, 17 of them are foreigners," Merrick said.

"And you ask me why Scotland can't qualify for a World Cup any more? It's pretty blatant.

"Foreigners that are coming into Scotland are very low standard.

"And that would happen here. You can only afford one or two decent ones.

"If you had seven (foreign) players the big teams would just flood them if they could afford them ... most of the other teams wouldn't survive."

The Australian Professional Football Clubs Association is pushing for each A-League club to be permitted six foreigners.

The association is an umbrella body for clubs who will take control of running the A-League under changes to the competition's governance.