It's unclear whether Kohli hurt his thumb while fielding or in the nets, but he had it dipped in a glass of ice while walking off the field at the end of India's training session in Southampton on Saturday.

from Times of India https://timesofindia.indiatimes.com/sports/cricket/icc-world-cup/virat-kohli-hurts-thumb-ahead-of-indias-world-cup-opener/articleshow/69618652.cms