Mullen had MRI scans this morning, which revealed damage to the lateral side of his right knee and that would likely miss the remainder of the Hyundai A-League season.

Making his first start in nine games having recovered from an ankle injury back in October, Mullen lasted just 32 minutes against the Heart after he went down in an innocuous incident in the Reds' penalty area.

"Daniel had an MRI scan of his knee this morning, which indicated he has damaged the lining of the outside of the joint and sheared off some of that lining," Adelaide United Doctor James Ilic said.

"He will have an operation on Tuesday and the surgeon will attempt to repair that, but he is unlikely to play again this season."

The prognosis of being sidelined for 12 weeks is devastating news for Mullen, who was determined to contribute to the Reds' run towards the finals having made 13 appearances this season.

Dr Ilic did not rule out the possibility that Mullen may be available if the Reds go deep into the business end of the season, but he admitted it was a highly unlikely scenario.

"It is certainly an injury that can happen and while it seemed innocuous at the time, his foot got stuck in the turf and his knee twisted and presumably it happened at that time," Dr Ilic said.

"Anything is possible but it is very unlikely. I think we're probably looking at three months before he is back playing, but he will be reviewed once he has had the surgery."

If the nature of last night's loss was not hard enough to stomach as Melbourne Heart scored twice in the last five minutes to snatch all three points, Adelaide United coach Rini Coolen is now faced with the fact that he will be without Mullen for an extended time.

"He is out for the rest of the season so that is really disappointing for us and especially for him," Coolen said.

Midfielder Adam Hughes slotted into Mullen's position in the centre of defence for the rest of the game against the Heart and proved he was more than capable of fulfilling the role, while Robert Cornthwaite could be a possibility to return from a minor hamstring strain.

The Reds begin a hectic two week period with a midweek encounter against Perth Glory on Wednesday.