United remain in the mix for the premiers plate but have been without Diamanti since he suffered a knee injury against Western Sydney on February 20.

"He's going in for arthroscopy tomorrow. I know the club haven't put it out there yet, but it only happened really yesterday that it was confirmed," coach John Aloisi told reporters on Friday.

"Because it's a bit of a mystery, we're not 100 per cent sure and neither is the surgeon.

"I really feel for him because he's wanting to be out there, he's wanting to perform for the club, for himself and for his teammates and he's not able to.

"But we've got his support and we hope it's nothing too serious and then see where it ends up after that."

Aloisi said not knowing what the injury was would have taken its toll on star playmaker Diamanti, while the 38-year-old hadn't yet spoken with United about whether he wanted to play on next year.

"They still don't know what it is, exactly. That's the hard part," he said.

"Sometimes when you have an injury and you know exactly what it is and you know your dates it's easier on you mentally and so when you don't know it plays on your mind.

"Mentally I can imagine he's probably not sleeping, he's probably thinking 'what do I have to do to get back?' and it's horrible. That's where we can only support him and then be there for him.

"I've been through it myself as as a player when you're not 100 per cent sure what your injury is and it just drags on and on and on and on and you're just 'am I ever going to get back to playing again?'

Diamanti has largely stayed around United's playing group but has also been given breaks away from the club.

Dylan Pierias has avoided a serious ankle injury but is touch-and-go to face Adelaide on Sunday while Adisu Bayew will have to prove his fitness after pulling up sore from Wednesday's clash with Newcastle.

Josh Risdon (syndesmosis) is unlikely to play as he targets a return for finals.

Australian Associated Press