The Central Coast Mariners have been dealt a major blow ahead of tomorrow's Hyundai A-League week 4 clash with Sydney FC, with news that goalkeeper Jess Vanstrattan has suffered a suspected ruptured Anterior Cruciate Ligament in his left knee.

Vanstrattan injured his left leg in freakish circumstances, as he routinely kicked during a drill at Mariners training this morning, and the former Juventus and Gold Coast United goalkeeper is awaiting the results of further tests before plotting his recovery with club medical staff.

"It's a huge blow for Jess and the club," said Mariners Head Coach Graham Arnold.

"He has fit in really well with the group since he arrived and was in great form, but when one door closes another one opens, and I have every confidence in Mathew Ryan, who will come in to the squad tomorrow."

Ryan, an 18-year-old prodigy who is highly rated by many of the country's top coaches including Mariners goalkeeping coach John Crawley, has been in top form for his NSW Premier League club Blacktown City following on for a breakout debut season in the National Youth League.

"He's a great young kid, has done really well in training and in the trial matches that he has featured in and I'm sure he'll do well," Arnold said.

"He'll do a great job tomorrow and looking further down the track we'll be looking to get someone in as an injury replacement to keep things competitive, but obviously first and foremost we wish Jess a speedy recovery, and hopefully he's back on the field as soon as possible," said Arnold.