Bob Bowman told The Associated Press on Sunday that Phelps has been taking part in workouts with the North Baltimore Aquatic Club. He's not yet committed to the sort of grueling program that would be needed if he decided to return to competition.
In fact, Phelps attended an NBA preseason game in China on Friday, visiting a country where he has extensive endorsement deals and remains extremely popular.
"He is occasionally training with the group to get back in shape," Bowman wrote in a text to the AP.
Phelps and Bowman remain close friends and business partners.
Initially adamant he would never compete again, Phelps softened his stance this past summer after reports began swirling of an imminent comeback. He told the AP during the world championships in Barcelona, "I don't know what's going to happen in the future. I don't know what's going to happen tomorrow."
When asked, yes or no, whether he'll compete at the next Olympics, Phelps coyly said he hasn't planned that far ahead in his life.
"I don't know. We're in 2013," he said. "There's nothing in the works right now."
Phelps long insisted he would quit swimming before he turned 30. After completing his brilliant career with a record 18 gold medals and 22 Olympic medals overall, he followed through on his long-stated goal of retiring after the 2012 London Games, where he was honored with a special ceremony after his final event.
He'll be 31 at the time of the opening ceremony for the Rio Games.
Phelps does have plenty of projects away from the pool, including a chain of swim schools and a foundation devoted to water safety. He is also passionate about golf and participated in a television series with famed coach Hank Haney.
But Phelps became much more realistic about his future on the links after joking not so long ago that golf was the only sport he'd possibly compete in at Rio, knowing it's returning to the Olympic program in 2016.
"It's probably the most humbling thing I've ever done, the most humbling sport I've ever done, the toughest thing I've ever done," he said.
Phelps still has plenty of time to resume serious training for Rio, though he would need to account for requirements such as re-entering the U.S. drug-testing program. He would certainly want to be back in peak condition in time for the next major international meet, the 2015 world championships in Russia, a key steppingstone to the Olympics.
Follow Paul Newberry on Twitter at www.twitter.com/pnewberry1963