While Fitzgibbon confirmed Braydon Trindall was on track to return for the first week of finals, the Sharks coach was less sure about Hiroti's prognosis.
Hiroti came off during Sunday's upset loss to Parramatta and joins fellow centre Jesse Ramien in the casualty ward for at least Saturday's clash with Melbourne.
"They're strange injuries, the rib ones. Some of them heal really quickly and you can move and get mobile quickly and then some take a little bit longer," Fitzgibbon said.
"So there's no time frame on that (his recovery)."
The injury has brought Riley Jones into the centres for his club debut, three years after playing his only other NRL game as a bench hooker at Newcastle.
With KL Iro rested, 21-year-old speedster Michael Gabrael makes his NRL debut on the back of impressive NSW Cup form.
Five-eighth Riley Pollard will also make his NRL debut as the Sharks rest playmaker Nicho Hynes, who has been battling a virus.
Utility Siosifa Talakai is an option to shift to the centres for next week's home elimination final if Gold Coast-bound Hiroti remains unavailable.
But Fitzgibbon said either of Jones or Gabrael could hold their spot with an impressive showing against the 11th-placed Storm.
"That's possible, for sure," he said.
"That's another reason we're having a look at these kids. We've (also) got Sifa there who's actually played semi-finals in the centres.
"I'll worry about that next week."
Fitzgibbon previously lamented his team had been playing "uptight" on the run to finals after dropping a third game in four starts last week.
But the coach said news of the debutants had helped loosen the Sharks up.
"It's been a sort up and down period last month ... the tension of that is you know what's ahead," he said.
"(But) excitement shifts when you've got three debutants."
Five-eighth Trindall had been in career best form before dislocating his shoulder against Canberra three weeks ago but is expected to partner Hynes from the first week of finals.
"All things going well and he gets through training, then we're looking to get him back," Fitzgibbon said.
The Sharks will celebrate the 10-year anniversary of their only premiership victory as they host 2016 runners-up Melbourne on Saturday night.
On the eve of another finals campaign, Cronulla welcomed almost all of their 2016 premiership team to training on Friday.
"We'll spend some time chatting with them and getting the players to get around them," Fitzgibbon said.
"There's always lessons in what a team's done previous to you."
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