He tried to play through the pain Sunday against Juan Martin del Potro in the semifinals. But the top-ranked Serb stopped during the second set, sending Argentina to the Davis Cup final against Spain.
"Not as serious as we have thought," Djokovic said Monday. "It's a partial rupture of a back muscle that deserves rest."
Djokovic skipped Friday's opening singles because of lingering back problems following his victory over Rafael Nadal in the U.S. Open final. But after losing the opening set 7-6 (5) against Del Potro, Djokovic fell from pain during the third game of the second set.
Djokovic said he may have to skip his next tournaments in China next month — the Shanghai Masters and China Open in Beijing.
"I hope I will recover by then," Djokovic said. "Anyway, I won't risk the worsening of the injury and that's why it is hard to predict for how long I will rest."
Djokovic said he has never had this kind of back problem.
"I carried the injury since the U.S. Open," Djokovic said. "It gradually got worse through the tournament, and I felt sharp pain in the finals."
"Luckily for me, I managed to hold on and to survive that fourth set and win the tournament," Djokovic added. "I came here quite late, and I could not get ready."
Djokovic beat Nadal 6-2, 6-4, 6-7 (3), 6-1 in the U.S. Open final for his sixth consecutive win over the Spaniard, including the Wimbledon final, and his third major title this season.
He entered the Davis Cup weekend with a 64-2 record. One of those losses came when he quit with a sore shoulder against Andy Murray in the Cincinnati final last month. Djokovic also lost to Roger Federer in the French Open semifinals.
Djokovic reportedly is planning to marry longtime girlfriend Jelena Ristic before the new season starts.