"It's not even once in a generation. It's once in 10 generations that someone like Michael happens to come along. ... I don't think we'll be seeing another Michael," said his coach, Bob Bowman, also the U.S. men's coach.
Phelps' routine before the start of a race has become so familiar to swimming fans that it's hard to comprehend that Saturday night was the final time.
He emerges from the waiting room in his hooded USA robe, headphones over his ears pumping classic rap tunes from Eminem or Young Jeezy.
He gives his starting block a shake to test its firmness, then wipes it down with a white towel.
He stretches his right leg, then his left.
Off come his sneakers, the robe and the headphones.
Then Phelps shakes out his arms and ascends the starting block.
Once in position, he whips his arms violently across his chest twice, smacking his shoulders with a loud crack each time. It's one of his signature warmup moves and maybe his last.
He has been adamant all week that this is the end, no matter what Ryan Lochte or his mother might say about Tokyo in 2020.
"Nope, nope, nope," he said Friday night when asked if he was reconsidering.
Phelps — as prickly a competitor as you'd ever find for most of his career — was almost chipper after finishing in a three-way tie for second in the 100-meter butterfly.
He beamed as he ducked in late for a press conference with the winner, Joseph Schooling of Singapore, and the other silver medalists, Chad le Clos of South Africa and Laszlo Cseh of Hungary.
Phelps seemed eager to hand the baton to a new generation.
"I'm excited to see just how much faster he can go," he said, nodding at Schooling, who set an Olympic record in beating him.
At these Olympics, a number of swimmers cited him as an inspiration. Photos circulated of him posing with Katie Ledecky, Japanese star Daiya Seto and yes, Schooling, when they were younger.
Phelps remembered the 2008 visit to Singapore on which he met the 13-year-old kid who would eventually beat him in his last individual race. It was a Team USA trip just before the Beijing Games. A band of monkeys had been stealing Power Bars from the U.S. camp, and Phelps was eager for the local schoolchildren to show the animalsto him .
Schooling recalled interrupting a Chinese lesson to bolt over and pose for a photo with Phelps. He wore glasses and came up to his hero's shoulder. Phelps sported an amusing Fu Manchu mustache.
"A lot of this is because of Michael," he said, clutching his new gold medal, the first ever won by an athlete from Singapore. "He's the reason I wanted to be a better swimmer."
As they walked around the pool deck after their medal ceremony, Schooling asked Phelps if he had four more years in him. Le Clos did the same, even though he and Phelps have maintained a rivalry at times. They want to keep swimming against the man they watched as children.
"No way," Phelps told them. "It's not going to happen."
Ledecky had a little fun talking about her fellow Marylander's false retirements.
"Last time was his last and this time was his last," she said, holding up two fingers to form air quotes. "I get to say I was on the team at Michael Phelps' last Olympics two times, which is cool. Maybe there will be a third time."
As someone who knows a thing or two about chasing and setting impossible marks, Ledecky said Phelps' medal totals will be challenged some day.
"Whether it's in 10 years or 50 years, I think somebody will eventually meet his records," she said. "But it might take a long time, and I know collectively, we're going to have to step up our game and really perform well to do what Michael has done for the sport."
Phelps has never spoken of his return for the 2016 Games as a quest to push his records out of reach. Rather, he sought the satisfaction of a job well and truly done.
"I didn't want to have a what-if in 20 years," he said. "I'm happy with how things finished."