The Spaniard entered the year's first grand slam tournament, the Australian Open, as one of the big favorites, but the reigning champion went straight out against American Mackenzie McDonald.
However, the 36-year-old world number two was visibly affected by the injury.
“I have been examined after the loss yesterday.
The MRI scan shows an injury to the hip flexor in the left leg.
Now there will be a rest from athletes.
Normal recovery time is six to eight weeks," writes Nadal on Twitter.