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

Scans have revealed the former Socceroo suffered a posterior capsule ligament sprain, which won't require surgery but will keep him sidelined for 10 to 12 weeks.

It means neither of the Mariners major off-season signings will feature in the club's closing matches as they attempt to avoid back-to-back wooden spoons.

Oar and Ross McCormack were expected to play key roles in coach Mike Mulvey's plans for 2018-19.

Instead Oar has made just 11 appearances in an injury-interrupted season while McCormack returned to parent club Aston Villa in December due to a knee injury having made just made five appearances for the Mariners.

Oar's career has stalled after being a part of Australia's 2014 World Cup squad and the successful 2015 Asian Cup squad.

The 27-year-old's last Socceroos appearance came in a World Cup qualifier in Bangladesh in 2015 and he missed selection for last year's World Cup in Russia plus this year's Asian Cup.

"I am putting my full focus into getting myself healthy and fully fit for next season," Oar said via a statement.

The Mariners face Oar's former club Brisbane this weekend with just one win all season and trailing the second-bottom Roar by five points on the A-League ladder.

Australian Associated Press