Konstas was called up after selectors dumped Nathan McSweeney after just three Tests, with the 19-year-old tasked with fending off Indian pace spearhead Jasprit Bumrah alongside an under-performing Usman Khawaja.
"He'll play Boxing Day," McDonald told reporters, referring to the teenager as "composed, relaxed".
"We have said from the get-go, leading into this summer, that we wouldn't shy away and age was no barrier.
"What he's shown is an array of shots and an ability to put pressure back on his opponents and he gets his opportunity.
"Really excited for him -- Boxing Day, the biggest stage."
Konstas will become Australia's youngest Test debutant since current skipper Pat Cummins took the field against South Africa at Johannesburg in 2011
It could be a baptism of fire, with an expected 90,000 fans at the Melbourne Cricket Ground and temperatures forecast to reach 40 Celsius (104 Fahrenheit) on Thursday.
Konstas thrust himself into the frame after becoming the youngest player since Ricky Ponting to score two centuries in the same Sheffield Shield match.
He also slammed a century playing for the Prime Minister's XI against India.
The five-match series is tied at 1-1 after India won by 295 runs in Perth before being crushed by 10 wickets at Adelaide. The rain-affected third Test in Brisbane was drawn.
Head is the highest run scorer in the series, plundering aggressive centuries at both Adelaide and Brisbane. But he picked up a minor thigh strain at the Gabba.
McDonald said he expected him to play but Head still had "some things to work through".
"I'm pretty confident he'll play," he added.
"He has been able to do his running. I think he will be fully functional come game time."
