"Doctors have told me that it is impossible to play, regardless of whether it hurts," said Nadal. "There is a significant risk of increasing the injury and I have no choice.
"I was well prepared to play and I had hopes of doing something important here."
Nadal, who has not competed since retiring against Croatia's Marin Cilic in Melbourne last month, was in Acapulco hoping to close the gap on world number one Roger Federer.
"I still don't know what it is, because we don't know. It seems it's not as bad as what I had at the Australian Open," the 31-year-old added.
"Now my main goal is to find out the extent of the injury."
(Reporting by Frank Pingue in Toronto. Editing by Nick Mulvenney)
