Arnold will undergo scans ahead of Friday night's match at Allianz Stadium after injuring her arm during training.
The Matildas are already without Ellie Carpenter (ankle) and Hayley Raso (thigh), the star duo ruled out earlier on Thursday after picking up injuries with their respective clubs.
Sermanni says Arnold had suffered the injury after a routine movement during training earlier in the week.
"She's doubtful (for Friday). That would be my best description," Sermanni said.
"I was actually watching it at the time and it was just a standard, average, straightforward dive.
"And suddenly she's done some damage to either a tricep or her shoulder."
It is unclear yet whether Arnold will be able to play in Monday's match at Newcastle's McDonald Jones Stadium.
Sermanni added Lyon defender Carpenter and Tottenham star Raso both entered camps with their respective injuries hopeful they would be fit in time for the two friendlies.
"Ellie came in with an ankle that we knew had some issues," Sermanni said.
"We were really confident with the information that we got, that her ankle would come in and settle down.
"Unfortunately, when you're travelling halfway around the world sometimes and you come in, that ankle just hasn't settled down, and it's just not worth the risk.
"She's got important games coming up in the Champions League.
"And with Hayley, it was something that she did in the final training session at Tottenham.
"She didn't feel it was too bad, and thought, 'I'll come in and it will be okay', but as I say, once we got the MRI we found it's not and we just can't take the risk."
Canberra United goalkeeper Sally James has been called up to replace Arnold, while fellow A-League players Alexia Apostolakis and Tegan Bertolissio fill in for Raso and Carpenter.
The Matildas are also without Katrina Gorry (ankle) and captain Sam Kerr (knee).
The injuries come as speculation swirls around the appointment of a full-time Matildas coach.
Carpenter's club coach Joe Montemurro has emerged as the frontrunner for the job, a role that has been vacant since Tony Gustavsson left his post after the Paris Games in August.
Former Arsenal and Juventus coach Montemurro had seemingly landed the perfect audition for the Matildas spot after being chosen to lead the A-League Women All Stars in an exhibition match against Arsenal in April last year.
The Melbourne product, who also coached Melbourne Victory and City, had thrown his hat in the ring to become the Matildas coach before moving to Juventus in 2021.
But Montemurro instead signed on with women's football powerhouse Lyon, six weeks before Gustavsson walked away from the Matildas.
Sermanni played a straight bat when asked about Montemurro succeeding him.
"That's somebody else's decision to decide who's going to replace me," Sermanni said.
"Joe's an outstanding coach ... You just look at his CV and he's an Aussie, so he's got great credentials for the job.
"But whatever way the decision that the FA are going, that's a decision that I'll fully support."
