Oar limped off in the first half of Sunday's A-League draw with Sydney FC after attempting an aerial challenge on Sky Blues' defender Alex Wilkinson.

Initial fears were that the 27-year-old had torn the ACL in his knee but coach Mike Mulvey said after the match it looked like a hyper-extension of the joint.

Oar was expected to undergo scans on Monday but youngster Aiden O'Neill was no clearer on the extent of the injury when he spoke to media after training on Tuesday.

"We all don't know yet what's actually happened," O'Neill said.

"From the video footage it didn't look too good."

Oar has endured an injury-ravaged first season with the Mariners with Sunday's match his first start for over a month, after knee issues kept him sidelined for almost all of November and December.

O'Neill said the winger's absence would hit hard as the Mariners desperately try to avoid back-to-back wooden spoons.

"Tommy is such a great guy, such a great player and brings a lot of experience and know-how about football.

"He'll be missed massively."

The Mariners, still with just one win for the season, face second-bottom Brisbane this weekend with five points separating the two teams on the ladder.

Despite the poor overall campaign, O'Neill believes the first team exposure he's had during his loan in Gosford will be crucial to making an impression when he returns to English Premier League club Burnley at the end of the season.

"You've got to go there and show that you've improved and you've gained experience and you can make a difference," the Olyroo said.

"It's all on me when I go back."