Both Rohit and Ishant are fighting to prove their fitness ahead of the Border-Gavaskar Trophy back in India.

Rohit injured his hamstring towards the end of the Indian Premier League earlier this month, and wasn't named in the Test squad originally.

The star opener was therefore not on the chartered flight to Australia, but has been adamant he will be fit to play in Adelaide from December 17.

However Shastri said the pure logistics meant if he wasn't on a plane this week, he would be unable to play in India's final warm-up game from December 11 due to COVID quarantine.

"He's going through some tests at the NCA (national cricket academy) and they're obviously going to decide how long he needs to take a break," Shastri told ABC Radio.

"But things could get difficult if he's asked to wait for too long, (because) then you're talking of the quarantine again.

"Which might make it really tough for even him to come just in time for the Test series.

"If you are to play in the Test series or any red-ball cricket you need to be in a flight on the next three or four days.

"If you aren't, it's going to be tough."

India are continuing their preparations while in lockdown, able to leave hotel quarantine to train ahead of Friday's opening one-dayer.

Rohit's fitness will be crucial for India, given he has averaged 65 in Test cricket over the past four years.

He has particularly been a thorn in the side of Australia in the limited-overs formats, without having the same success against them in Test matches.

India will also rely on him even more heavily this summer, given Virat Kohli's absence after the first Test.

Meanwhile Ishant has played just one competitive match since India's tour of New Zealand in February, after he hurt his abdominal muscle in the IPL.

The quick is back bowling long spells, but faces a similar situation to Rohit to land in Australia in time for the first Test.

Australian Associated Press