The World number nine is unable to take part in the Cincinnati Masters or the US Open Grand Slam, which begins on August 28.
“Kei hit a serve during practice and heard a ‘pop’ in his wrist,” BBC Sport quotes Nishikori’s manager as saying.
“At this stage, we have elected not to do surgery and Kei is in a cast. After the swelling comes down in the next weeks, we will evaluate next steps.”
The Japanese joins Novak Djokovic and Stanislas Wawrinka in missing the US Open.