The world number three has not played since suffering a shock second-round Wimbledon defeat to Czech Lukas Rosol in June. He missed the Olympic Games, where he was the defending champion, and was forced to withdraw from the US Open.
"I am feeling the calm that comes from knowing I'll be back when I genuinely feel ready to play at the level I really want to and not have to suffer through every match," Nadal told Spanish television.
Asked if he would be fit for the Sept. 14-16 Davis Cup tie in Gijon, when Spain continue their bid for a fourth crown in five years, he added: "I don't know".
"The truth is I really don't have any idea and when the time comes my knee will decide and if the knee is fine then the captain will decide," the 26-year-old said.
Due to his physical style of play, Nadal has suffered a string of injuries during his career, including a foot injury in 2005 and more recently problems with his knees.
Spanish tennis federation (RFET) doctor Angel Ruiz Cotorro said last month Nadal was suffering from a condition known as Hoffa's syndrome, a swelling round the tendon just below the kneecap in his left leg.