The Indian captain will return home post the first Test in Adelaide.
Kohli will return home post the first Test in Adelaide to be with his wife Anushka Sharma as they await the birth of their first child in early January. With the quarantine regulations in place amid COVID-19 pandemic, there is no chance for Kohli to return and take part in the last couple of Tests.
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"The birth of your child is a very important time and he wants to be there for it and support his wife. So, I can understand," McGrath was quoted as saying by Sportstar.
"There is no doubt that it will impact the series, for India to lose a player of his quality and class for potentially three of the four matches is a big blow. It's up to the other players to do the heavy lifting when he is not there."
Not just Rohit, McGrath thinks Kohli's absence shall inspire all the Indian batsmen to show their mettle on one of the toughest Test tours possible.
"Rohit Sharma is a quality batsman who hasn't achieved at Test level what he should be achieving in my opinion... Maybe when Virat goes home, he might step up."
"Having said that, you can't just focus on one player. You have (Ajinkya) Rahane, (Cheteshwar) Pujara, (KL) Rahul, so there's a quality batting line up. Once Virat leaves, it will be an opportunity for someone else to put their hand up and make a mark on the series. Maybe that could be Rohit Sharma."
India's task for Melbourne, Sydney and Brisbane will be slightly more comfortable if Kohli can stamp his authority in Adelaide. However, that won't be easy at all, considering the venue will be hosting India's maiden away D/N pink-ball encounter. No Aussie team has yet lost a pink-ball outing in home conditions.
"The first Test is going to be interesting - it is a Day/Night Test ... India haven't played a Day/Night Test in Australia before and it is unique, especially with the sun setting and at night, where it starts favouring the bowlers, primarily the quicks. It is about timing - when you are bowling or batting in those conditions," said McGrath.
"Virat, if he is going to have an impact, needs to come good in that Test match... the first Test will set the tone - two years ago, when they (India) were here, they won the first game in Adelaide and then, went on to win the series. So, both teams would want to get off to a good start."
After Kohli, India's second most impactful cricketer for the Test leg will be pacer Jasprit Bumrah. The renowned quick was outstanding when India rewrote history books on their last trip in 2018-19 summer, winning the Test series 2-1 for the very first time on Australian shores.
"(Jasprit) Bumrah is a class bowler. I have spoken to him, and I like his mindset, his attitude and the way he goes into a game. He has a lot of mental strength to go along with his skill," said McGrath.
"He bowls good pace and good areas... his second and third spells are just as quick as his first, so he has got every attribute to be one of the greats at the end of his career."
"He will be fine - it is about confidence and getting a few overs under your belt... the first few matches here will be important for him in terms of finding the right length on the slightly quicker wickets here."
"That said, he did pretty well in the IPL recently, bowled a lot of overs, so he will be confident in his body," he signed off.
The Test series begins on December 17.