Djokovic said the shoulder had been bothering him for quite some time.
“The pain was constant for weeks now,” he said. “Some days higher, some days with less intensity and obviously taking different stuff to kill the pain instantly. Sometimes it works, sometimes it doesn't.
“You just know (it is too much) when you know, I guess, when you feel like you're not able to hit the shot anymore. It is frustrating, very frustrating.”
Djokovic said he would evaluate the injury in the coming days and hopes to play his next event in Tokyo, in September.
“I just hope I'll get a chance"
But despite the injury, the Serb said 2019 would go down as a good year, having added another Australian Open and another Wimbledon title to take his grand slam tally to 18.
“I won two slams out of four and played semis in French,” he said. “I had a really good grand slam season. I cannot complain, not at all.
“There are plenty of big tournaments ahead. Rankings-wise, plenty of points to defend for me and try to withhold that ranking No 1. Obviously Rafa has been playing well and Roger and the other guys.
“I just hope I'll get a chance to be competing, because once I'm healthy, I actually like my chances playing in Asia. And also indoor season, I play historically pretty well in those last couple months of the year.”