The Spaniard, who hasn't played since losing in the second round of Wimbledon in June to little-known Lukas Rosol, is recovering from a partially torn patella tendon in his left knee.
In interviews with European newspapers Tuesday, the 11-time Grand Slam champion said he's not sure how soon he'll be back or whether he'll play in the Australian Open in January.
"All that is in my mind is to keep working hard to come back," Nadal said. "I cannot think about the future because it's not like if you break your arm and you know you will have a few weeks like this, then a few weeks like that and then you are back.
"This is a day-by-day thing. I have checks every day to see how I'm improving. I can't predict what will happen."
Nadal could miss the ATP World Tour Finals in London Nov. 5-12 and Spain's Davis Cup final against the Czech Republic Nov. 16-18.
As for the Australian Open, he was quoted in the Daily Mail as saying: "I hope you see me in Australia. That is the biggest goal for me, to come back just before then in Qatar, but I cannot say for sure it is going to happen."
"The only thing is to recover well. I want to be 100 per cent when I come back," he said.
Nadal said he played with anti-inflammatories to get through the French Open, which he won for a record seventh time, and took pain-killing injections at Wimbledon.
He said he was particularly upset at missing the London Olympics, where he had been scheduled to carry the Spanish flag at the opening ceremony.
"I was very, very sad for three weeks around then," he said. "It only comes every four years. Missing the U.S. Open was hard but you think you will have more chances."
Nadal praised Andy Murray, who won the U.S. Open for his first Grand Slam title after losing in four previous major finals.
"He deserves it more than anyone," Nadal said. "I am very happy for him."