Djokovic said that it was a very quick move that caused the pain. Despite that though, he said that he was able to serve again with minimum complications which is positive for him. He can now achieve his personal and professional goals for 2020.
"It was, again, a very quick move that caused pain. But in the third set, I was able to serve, I was able to play with no pain, which was positive."
https://twitter.com/DavisCupFinals/status/1197899854573387777?s=20