Cadman, 22, has been ruled out for at least the first two weeks of the season as he deals with ongoing pelvic soreness.
It was disappointing news for Cadman, who is widely expected to become a genuine star after kicking 44 goals last year.
"It is a tricky injury," Giants coach Adam Kingsley said on Friday.
"It's lower back, one that you kind of think will settle, but you're never quite sure. It can settle faster than what you expect too.
"So we'll be pretty gentle with him. We're not going to push him in this part of the season to get back before he needs to.
"It's probably one that has sprung on us over the last week, we weren't expecting it.
"He has done a full pre-season and been quite good in it, so it's a bit of a surprise one.
"But just take them as they come - and hopefully they don't come too often."
The Giants have a mounting injury list already.
Tom Green (ACL) is out for the season while there is no confirmed return date for Josh Kelly (hip) or Darcy Jones (ACL).
Hamstrung star backman Sam Taylor will miss the first four-to-six weeks and Brent Daniels (hamstring) is up to a month away.
But Toby Bedford (hamstring), Leek Aleer (groin) and Toby McMullin (hamstring) aren't far off playing.
"I haven't really paid a heap of attention to any doom and gloom externally," Kingsley said.
The Giants will at least be able to call on dynamic midfielder Finn Callaghan, who has overcome a minor hip flexor injury in time to pair with Toby Greene and new recruit Clayton Oliver.
Kingsley said Callaghan had built his training loads over the past three weeks.
"We're picking him with confidence. We're not sort of rushing him if that's the question," he said.
"But he's obviously a really important player for us, strengthens our midfield."
The Giants won't lean into the heartbreak Hawthorn dealt them in last season's elimination final when they open their new AFL campaign against the same opponent on Saturday.
"There's nothing that we can do about that right now," Kingsley said.
"Winning opening round against the same opposition doesn't neutralise what happened last year, so I think that's a myth to suggest otherwise."
Kingsley had no concerns over the Engie Stadium surface, which hosted two Bad Bunny concerts in recent weeks.
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