The concussion sub came to their rescue.
Jadeja was hit on the helmet in the final over after top edging the ball into his helmet.
However, he received no medial treatment or assessment until after walking off the field at the end of the innings.
With Jadeja ruled out with concussion, 12th man Chahal got the call up as a fortuitous bowling option.
He made a huge first impression, picking up two wickets in his first two overs, to turn Australia's run chase on its head.
He finished with 3/25 from four overs, having picked up the big wickets of Aaron Finch, Steve Smith and Matthew Wade.
Australia never recovered and finished 11 runs short at 7/150 as India took a 1-0 lead in the series.
Chahal was named man of the match, just to rub salt into Langer's wounds.
Fox Cricket commentator Adam Gilchrist said Langer would not have been happy to see the concussion sub make such a big impact on the contest.
Langer appeared to be seething when speaking with match referee Boon in a conversation that many have declared was a heated exchange about India's move.
Gilchrist further explained why Langer would have been asking questions to Boon.
"I assume the Australian management would be saying 'I'm sure there would be protocols that need to be met to make that judgment'," he said.
"Do the test, because he scored a lions share of his runs after that."
Waugh added: "You can't have it both ways. You can't have a substitute who can then bowl for you and then bat on."
The reaction from across the cricket world was sceptical of Jadeja's condition.
Former English captain Michael Vaughan posted on Twitter he could smell a rat.
"No Doctor or Physio came out to Test Jadeja for concussion ... he then looks like he has done something to his leg ... then they pull the concussion replacement .. !!!!! #RatSniffed," Vaughan wrote.
Aussie Test star Usman Khawaja called the tactic "cagey" from the Indians.
Indian Test great Aakash Chopra cheekily posted on Twitter the switch gave India the "best of both worlds".
It is a headache Langer predicted four years ago when discussing the introduction of concussion subs in 2016.
He admitted he had doubts teams would "respect" the spirit of the rule.
"The problem is when people start looking for loopholes. Then it's no good, it wrecks it for everybody," he said.
"We don't have runners any more but if people had maintained the integrity of that then we'd still have runners."
Aussie cricket great Tom Moody pointed out on Twitter that India should have assessed Jadeja immediately and treated the incident as serious when he was struck on the helmet — instead of waiting until the end of the innings.
"I have no issue with Jadeja being substitute with Chahal. But I do have an issue with a Doctor & Physio not being present after Jadeja was struck on the helmet which I believe is protocol now," he posted on Twitter.
His sentiments were echoed by former Australian player and ABC radio commentator Michelle Goszko.
"The ball did whack him on the badge of the helmet, but there was no physio or doctor that came out to check on him," she said.
"Normally there's a concussion assessment done on the ground but there wasn't."
India announced during the break that Jadeja was being assessed by medical staff and on Saturday morning was ruled out of the remainder of the T20 series. Shardul Thakur was named as his replacement.
