Australia's goalkeeping stocks were battered even before the tournament began, and the Matildas are still counting the cost.
Lyon custodian Teagan Micah was ruled out of the Asian Cup due to concussion, Arnold had a calf niggle, while Jada Whyman pulled out of the tournament on Sunday with a knee injury.
It meant fourth-choice keeper Chloe Lincoln, who wasn't even in the original squad, was thrust into the starting line-up for Sunday's 1-0 win over the Philippines in the tournament opener.
Australia face Iran on the Gold Coast on Thursday before rounding out their group stage against South Korea in Sydney on Sunday.
Although Arnold is no guarantee to play in either of those matches, Montemurro is backing the 2023 World Cup hero to feature again soon.
"Mackenzie's OK. She just had a little bit of calf fatigue. It was just a little bit tight," Montemurro said.
"We just felt that giving her a little bit more time would be better.
"We've got to grow into this tournament, and not just from a football perspective, but also in terms of player recovery and making sure that we keep everyone healthy from that perspective.
"So we just made a decision with Macca."
Montemurro said Whyman, who was set to make her international debut against the Philippines, injured her knee during a freak training accident.
"She's come back from a knee situation, and unfortunately she made a save at training and fell on it," Montemurro said.
"And it was just one of those freak occasions that happens. I feel sorry for Jada. We're with her."
Lincoln also expressed her sorrow at Whyman's misfortune.
"I was pretty devo about Jada too, because I know this would have been awesome for her, for her to make her debut," the 21-year-old Brisbane Roar goalkeeper said.
"My heart goes out to her."
Lincoln, who just a week ago was focusing completely on the A-League Women season, said playing in the Asian Cup had come as a big surprise.
"It all happened so quickly, so there wasn't much time to dwell on it," she said.
"But I feel very lucky to have a good group around me who's so supportive.
"Macca spoke to me before the game as well, and she was just like, 'Keep it simple, work yourself into the game, no pressure to do anything crazy'.
"We controlled the game. I didn't really have to do too much."
Sam Kerr's 14th-minute header won Australia the game.
But the Matildas looked the most dangerous following the introduction of Mary Fowler in the 67th minute.
Fowler, playing just her third game since recovering from a knee reconstruction, caused the Philippines all sorts of headaches as she set up numerous opportunities for her teammates.
Montemurro said his decision to play Fowler off the bench was a "technical decision" and nothing to do with managing her loads.
"Mary's fine," Montemurro said.
"I just made a technical decision not to start her because she hasn't really done a lot of the work we've done over the last few camps.
"This is the first real camp that she's been on the pitch and working with us.
"So I just felt it's better that she's integrated slowly. But there's no monitoring, there's no modification. She's ready to go."
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