5th February 2018, 16:55 | Leeds
The 26-year-old former Gold Coast Titans prop, who joined the Rhinos on a three-year contract in the winter, went on as a substitute in their opening 16-12 win at Warrington Wolves last Thursday but is now facing a spell on the sidelines.
The New Zealander will definitely miss Thursday's home game against Hull KR, which is being played at Elland Road while reconstruction work continues at Headingley, and is a major doubt for the World Club Challenge clash with Melbourne Storm at AAMI Park on Friday week.
"He came off with a busted bicep," McDermott told a news conference at Elland Road. "We're having it checked out to see the extent of that.
"He won't play on Thursday. If the surgeon says he can carry on without have it operated on, he'll probably still have two or three weeks off to let it settle down.
"If he has to have it operated on, it will be 12 weeks out."
The champions fly out to Melbourne on Friday and McDermott says Peteru may yet make the trip.
"It's unlikely but I wouldn't rule him out. We'd take a maybe with us," he added.
The good news for the Rhinos is that second-rower Stevie Ward is set to make his comeback from shoulder surgery on Thursday and there are hopes that prop Mitch Garbutt could take the place of Peteru after missing the opening game with a damaged quad.
Ward underwent surgery three days after helping his team win the Grand Final and resumed full training just over a week ago.
"He's got to get through a last couple of checks but, all being well, he will play," McDermott said.
Despite being denied home advantage, McDermott is delighted his side get the chance to play at Elland Road, where the club are expecting a crowd of around 16,000.
"To play at a stadium like this is pretty special for us," he said. "It gives us that big-game feeling before next week as well."
Meanwhile, McDermott says the club owe a debt of gratitude to former Rhinos captain Danny McGuire, who deserves a "brilliant" reception when he returns to Leeds as a Hull KR player.
McGuire, Super League's all-time record tryscorer, ended a glorious 17-year career with his hometown club after leading them to victory over Castleford Tigers in October's Grand Final and McDermott is expecting him to pose a threat to his side on Thursday.
"Certainly trying to plan against him is a difficult task," McDermott said. "When he gets it right, sometimes he's unplayable, as he showed in the Grand Final.
"There is still lots of life left in Magsy and I'm sure he'll come up with a big one on Thursday night.
"He should get a brilliant reception. I can't imagine any of our fans thinking bad of him because he's gone to Hull KR. It was the right time for us to put faith in some other people and the right time for Magsy to move on.
"He left with our best wishes and he left in the right form and in the right tone. Our club should be indebted to Danny McGuire for many years to come and I'm sure our fans will treat him with the utmost respect."