Sinner announced on Tuesday that he will miss the rest of the year due to knee inflammation.
The five-time major winner has not played since July when he beat Alexander Zverev in the Wimbledon final. He then withdrew from the US Open because of a right-knee injury.
"I haven't been in touch with Jannik since he got injured, I just saw the news two days ago," Djokovic said on Thursday at the Shanghai Masters where he hopes for double success after he beat Alex de Minaur in Beijing on Tuesday for his 102nd career tournament triumph at the China Open.
"So I'll reach out to him to see how he's doing, as someone that has been through some major injuries in my career."
For so long an ever-present on the ATP men's tour, Djokovic made the decision in late July 2017 to sit out the rest of that year after struggling with acute pain in his right elbow.
"I had a surgery, and it was the only big surgery that I had," the 24-time major champion Djokovic said. "So I'm aware of how difficult that is making that comeback."
The 39-year-old Djokovic said the process extended beyond the physical healing.
"Then it's mental, you know. You're on the court and you're thinking, 'am I really completely healed and free to make this run?'" he said. "I mean, for him if it's a knee, obviously it's affecting everything, it's affecting every single movement."
Sinner had already announced he would miss the Shanghai Masters. He'll also miss the Paris Masters in November and the ATP Finals later that month, ensuring he will definitely lose his top ranking to Alexander Zverev.
Worse still, he won't be able to play for hosts and three-time defending champions Italy in the Davis Cup.
"You know for sure that tennis misses him," said Djokovic, who faces Hubert Hurkacz in the second round in Shanghai.
Sinner had a 30-match winning streak this year before his meltdown in a second-round defeat at the French Open.
