Nadal, 31, pulled out of the Paris Masters before his quarter-final match last week after experiencing discomfort in his right knee during his third-round win over Pablo Cuevas.
It raised fears the 16-time grand slam champion would join a lengthy list of leading names, including 2016 champion Andy Murray and three-times London winner Novak Djokovic, who are already sidelined for the season-ender.
Moya had better news though.
"Rafa is fine, the knee is okay," the former world number one said in Milan after taking part in a coaching workshop at the Next Gen ATP Finals.
"He stopped just to take no risks. Rafa will be competitive in London because it's the only big tournament he hasn't won."
Nadal silenced the naysayers this year by returning from an injury-plagued 2016 to win the French Open for a 10th time before taking the U.S. Open crown.
It helped him secure the year-end number one ranking for the fourth time, nearly a decade after he first achieved it.
Moya, who began working with his fellow Mallorcan a year ago, said Nadal's feats this year had not surprised him.
"I expected Rafa to be number one again," he said. "I'm not surprised because I've seen him training as usual with lots of passion. His tennis and his mind has taken him to number one."
Nadal is due to start his ATP Finals challenge on Monday against Belgium's David Goffin and also faces Bulgarian debutant Grigor Dimitrov and Austrian Dominic Thiem in his group.
(Reporting by Martyn Herman; Editing by Ken Ferris)
