"I really want to be back competing and enjoying the tennis tour, but I have many years in front of me and my knee needs some rest," Nadal said in a statement on Monday.
"I will be back when I have no pain and am able to compete without problems."
Nadal has been having treatment for a partial tear of the patella tendon and an inflammation of the Hoffa's fat pad of his left knee.
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 feel better after the meeting with my doctors and happy knowing that the evolution of the past weeks has been positive and surgery has been avoided," Nadal added.
"I will continue to work with the plan my doctors have established to be able to compete as soon as possible in the right conditions.
"I would also like to thanks all for the support I have been receiving."
The statement said doctor Angel Ruiz-Cotorro was to hold a news conference in Barcelona on Tuesday to give further details.
Spain, who are bidding for a fourth Davis Cup crown in five year, take on the US in Gijon, northern Spain, on September 14-16.