Nadal had been a major injury doubt for the end-of-season tournament with a knee problem, and the world number one was visibly hindered as he succumbed to a 6-7 7-6 4-6 defeat to Goffin at London's O2 Arena on Monday night.
After the match, Nadal soon announced that he was pulling out of his remaining matches, but the world number one is hopeful that he will return for the start of the new season in the New Year.
The 31-year-old told reporters: "I am off and the season is finished. I am really not ready to play. I really fought a lot during the match knowing probably it was a big chance [that it would be] the last match of the season. It's about the pain and I cannot hold with enough power to keep playing.
"The good thing is [the injury] is nothing new. We hope to have the right rest, the right work and try to be ready for the beginning of the next season."