The 2008 gold medalist withdrew from the French Open with a wrist injury and missed Wimbledon.
“I will not be at the best level in any of the categories,” Nadal told journalists after arriving in Brazil on Sunday. “I have not competed for two months and I have not trained a lot.”
“I will train a few days here to see what I can do and then decide what is best, to be more positive for me and the team,” he said.
Nadal’s Spanish team mate David Ferrer acknowledged last week that the 14-time grand slam champion’s fitness situation was “delicate”.
World number five Nadal, who is due to be Spain’s flag bearer, was absent from the 2012 Olympics due to injury and should he withdraw, it would come as a significant blow for tennis at the Games with several other high-profile player also missing.
Roger Federer sits out due to a knee problem and Wimbledon finalist Milos Raonic withdrew citing fears over the Zika virus.