"The diagnosis was confirmed based on a clinical assessment in the dressing room during the innings break by the BCCI Medical Team," the board said in a statement. "Jadeja remains under observation and will be taken for further scans if required based on the assessment on Saturday morning."
Seamer Shardul Thakur, who wasn't originally in the T20I squad, has been added to the group as Jadeja's replacement.
ALSO WATCH: Match highlights - Jadeja, Chahal, Natarajan help India win series opener (Indian subcontinent only)
Jadeja had batted with a hamstring strain, which was apparent, and then in the last over of India's innings, he top-edged a rapid Mitchell Starc delivery onto his helmet. The blow was hard enough to ricochet to backward point, where the catch was dropped.
Jadeja wasn't given a concussion test on the spot, but was subsequently replaced by Yuzvendra Chahal at the innings break amid controversy. Chahal took the key wickets of Aaron Finch and Steven Smith on his way to match-winning figures of 3 for 25.
In the absence of Jadeja, Chahal and his Royal Challengers Bangalore team-mate Washington Sundar are the only two frontline spin-bowling options for India in the remaining two T20Is. Without Jadeja's power, India's lower-middle order also appears lighter, with Sundar likely to slot in at No.7.
As for Thakur, he enters the T20I squad with encouraging form, having taken 3 for 51 in the third ODI, which India won in Canberra. He was also Chennai Super Kings' third-highest wicket-taker in IPL 2020 with 10 strikes at an economy of 8.50.
India T20I squad: Virat Kohli (capt), KL Rahul (wk), Shikhar Dhawan, Mayank Agarwal, Shreyas Iyer, Manish Pandey, Hardik Pandya, Sanju Samson (wk), Washington Sundar, Yuzvendra Chahal, Jasprit Bumrah, Mohammed Shami, Navdeep Saini, Deepak Chahar, T Natarajan, Shardul Thakur