Rohit participated as an Impact Player due to 'mild back stiffness'

Piyush Chawla explained that it was a precautionary measure.
Rohit Sharma had to play as an Impact Player for the Mumbai Indians against the Kolkata Knight Riders on Friday owing to a "mild back stiffness".
"He just had a mild back stiffness, so it was just a precautionary thing," Piyush Chawla stated at the post-match news conference.
MI struggled to chase down a 170 mark on a two-paced Wankhede pitch, with Rohit scoring 11 from 12 balls. It was their eighth loss in 11 games, putting them in ninth place with playoff chances hanging by a thread. Chawla was honest when asked what MI still had to play for.
"For pride and reputation," he continued, "because sometimes when you enter the field, you don't know if you're going to qualify or not." You must play for your name, and that is exactly what we are doing.
Chawla's silver lining throughout the match was that he became the IPL's second-highest wicket-taker, surpassing Dwayne Bravo's tally of 183.

Chawla got there after catching and bowling Rinku Singh's first delivery of the match. When asked about his career, Chawla said: "It has been a beautiful journey because IPL started 17 years back and at that time nobody gave that much preference to the spinners, but now if you see that in the top few wicket-takers from India are all the spinners, [R] Ashwin, Yuzi [Yuzvendra Chahal], myself, so it's a good feeling."