Siraj's late call-up comes on the back of fresh injury concerns for Bumrah, who was required to undergo scans on his lower back in Bengaluru before the National Cricket Academy's medical staff takes a final call on his participation in the T20 World Cup.
Siraj last featured in a T20I for India in February against Sri Lanka and is seen mainly as a red-ball option at present. His latest outing was a one-off appearance for Warwickshire against Somerset earlier this month, where he picked up a five-wicket haul.
So far Siraj has played only five T20Is, picking up as many wickets at an economy of 10.45. Since 2020, however, he's been a regular feature for Royal Challengers Bangalore, and was also one of three retentions for them ahead of this year's auction.
Siraj isn't the only late addition to the squad. Just before the series opener, Shreyas Iyer, Shahbaz Ahmed and Umesh Yadav were also included in the side.
Iyer came in for the injured Deepak Hooda, who is also under the supervision of the BCCI's medical staff at the NCA, while Umesh filled in for Mohammed Shami, who is recuperating from Covid-19 having returned a negative test. Shahbaz, meanwhile, replaced Hardik Pandya, who is undergoing "conditioning-related work" at the NCA.
The Guwahati T20I on Sunday will be followed by the final match of the series in Indore on Tuesday. The World Cup-bound squad is expected to depart later that week for Australia, where they will have a short camp in Perth. The selectors are likely to announce a second-string squad, led by Shikhar Dhawan, for the three-match ODI series against South Africa.
India squad for 2nd and 3rd T20Is: Rohit Sharma (capt), KL Rahul (vice-capt), Virat Kohli, Suryakumar Yadav, Rishabh Pant (wk), Dinesh Karthik (wk), R Ashwin, Yuzvendra Chahal, Axar Patel, Arshdeep Singh, Harshal Patel, Deepak Chahar, Umesh Yadav, Shreyas Iyer, Shahbaz Ahmed, Mohammed Siraj
Shashank Kishore is a senior sub-editor at ESPNcricinfo