St George Illawarra forward Korbin Sims has been ruled out for up to eight weeks after suffering a broken arm in the Dragons' NRL win over Newcastle.
Sims copped the injury on a routine carry not long after coming on midway through the first half at McDonald Jones Stadium on Sunday.
The 27-year-old bravely played through the pain for the remainder of the first half before club doctors ruled him out of action at halftime.
The injury comes in just his second game for the Dragons after completing a two-game suspension incurred while playing for Brisbane last year.
"Korbin will undergo surgery on Monday afternoon to stabilise the fracture," Dragons head of athletic performance Nathan Pickworth said on Monday.
"His return to play will be approximately six to eight weeks."
McGregor admitted after the game that doctors were initially unaware of the severity of the injury despite Sims relying on his right arm to attempt tackles.
He also turned the ball over on a simple carry close to the line.
"Certainly Korbin was displaying to the medical staff that he'd like to stay there and try and work through it," McGregor said.
"Then at halftime the doctor had a look at it and then the choice was made that he's not to return to the field."
Sims isn't the only injury concern for the Dragons with second-rower Jacob Host also failing to finish the match against the Knights after he aggravated an ankle issue.
He came off with about half an hour to play and didn't return.
Losing either or both players for any length of time will hurt a side already missing star Jack de Belin due to the NRL's no-fault stand-down policy.
"He rolled it leading into round one, didn't do much training that week, and it's a recurrence of that," McGregor said of Host's injury.
"I'm not a doctor or a physio but he came off because he couldn't continue.
"We'll just have to look at that during the week."
The Sims blow means McGregor, who must also make do without skipper Gareth Widdop, may have to rely on Luciano Leilua or Mitchell Allgood against Canterbury on Sunday.
Australian Associated Press