The visitors' most experienced pacer has suffered an injury before the all-important Test tour.
Ishant, who has been on four Test tours down under, is in rehab for abdominal muscle tear since returning prematurely from IPL duties in UAE earlier this month.
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While not named in the squad announced on Monday (October 26), the 32-year-old's progress is currently being monitored at the NCA in Bangalore and he can join the touring contingent later.
"I am predicting Australia to edge out India in this series," Gillespie was quoted as saying in an interview by Hindustan Times. "I think home advantage is going to be a factor. Besides, it will be a blow with both Ishant Sharma and Bhuvneshwar Kumar injured. Bumrah and Shami have been bowling well for India. But, India will miss Ishant for his experience in Australian conditions."
Ishant has been tremendous for India over the last five years, especially on away tours. He took 11 wickets in 3 Tests at an average of 24 on India's last trip to Australia where Virat Kohli's men pulled off country's maiden Test series win on those shores.
With 297 wickets in 97 Tests, Ishant is also just three more games away from becoming only the second Indian fast bowler since Kapil Dev to play 100 Tests.
"He has been bowling beautifully in the last few years. You only have to look at his results to show why he has performed so well,” said Gillespie, who coached Ishant during his county stint with Sussex in 2018.
"He also brings a subtle difference to the bowling attack. It’s not often that a right-arm seam bowler comes wide of the crease and bowls inswingers to right hand batsmen and across left-handers."
"It’s not a common angle for batsmen to face. You normally have right-arm bowlers looking to get close to the stumps and look to swing it away from the right-handers and bowl it in to left-hander."
Gillespie, among the finest pacemen that Australia produced, thinks quite highly of the current pace battery that his country possesses, complemented wonderfully by off-spinner Nathan Lyon.
“When I look at the Australian bowling line up: Mitchell Starc, Pat Cummins and Josh Hazlewood - it is the best seam attack Australia has ever produced in the history of Test cricket. They make a wonderful combination."
"Obviously, with Nathan Lyon, our best off-spinner, it is a pretty formidable line-up and it’s going to be a real challenge for India in Australian conditions," he said.
In order to match Cummins and Hazlewood in their own den, Gillespie thinks, the likes of Mohammad Shami, Jasprit Bumrah and Ishant, if recovered, will have to be at their best, a task only made tougher from the last tour by the return of Steve Smith, David Warner and the emergence of Marnus Labuschagne.
"For the Indian seam attack, the real challenge is how quickly they can adapt to Australian conditions and get their length on delivery right. Sometimes, when teams come from the sub-continent, it takes a little while to find the correct length. The Australians do it year in, year out,” said Gillespie.
"I know Australian fast bowlers are going to bowl that fuller length, and challenge the front-foot defence of the Indian batsmen. With India having a powerful batting line-up, it’s going to be an exciting series. Everyone is looking forward to seeing these two giants do battle, in what has been a challenging year for everyone," he concluded.
The four-Test Border-Gavaskar Trophy begins on December 17 with India's first away pink-ball D/N encounter.