His image was riding high after he won a record eight gold medals at the Beijing Games over the summer.
Bit the photo wound up in a British tabloid Sunday, forcing Phelps to publicly apologize and his handlers to deal with sponsors whom observers say are surely none-too-pleased about the swimmer's choices away from the pool.
"I engaged in behavior which was regrettable and demonstrated bad judgment," Phelps said in the statement released by one of his agents. "I'm 23 years old and, despite the successes I've had in the pool, I acted in a youthful and inappropriate way, not in a manner people have come to expect from me. For this, I am sorry. I promise my fans and the public it will not happen again."
That probably won't be enough to assuage current and potential sponsors said Robert Tuchman, president of New York sports marketing agency Premiere Corporate Events.
On The Early Show Monday, Tuchman told co-anchor Maggie Rodriguez the Phelps statement, "That's kind of (the), 'Hey, I'm a young kid, and look, we all do things" ' excuse.
Tuchman says the photo is likely to hold Phelps back "in the endorsement game. ...It's going to really hurt him in terms of his marketability now."
Phelps, said Tuchman, "is trying to make this go away as fast as possible."
Speedo issued a statement of support, but Tuchman says, "They've got a lot of money invested in him right now. And a lot of these companies, the usual tactic is, 'Let's support our person until we see what the effects are. ... What is the public opinion going to be? Can this go away in a few months?' And if it doesn't, then you'll start seeing companies that are endorsing him pulling out."
Tuchman added, "My prediction is that, with a story like this, that a few years from now, when, you know, we're all gearing up for the Olympics in London, you'll see a lot of people who are going to be jumping back on the bandwagon, providing he stays out, you know, of this type of negative light.
"But I do think that a lot of companies now that might have or might be in some economic trouble could use this as a way out of basically paying him these millions of dollars.
"Certainly, Phelps ... had everything. He was extremely marketable. ... This was the Golden Boy or the golden image. And Olympics and marijuana don't mix."
After the 2004 Athens Games, an underage Phelps was arrested for drunken driving, pleaded guilty and apologized to his fans, saying he wouldn't make the same mistake again.
"Michael is a role model, and he is well aware of the responsibilities and accountability that come with setting a positive example for others, particularly young people," the U.S. Olympic Committee said in a statement reacting to the photo. "In this instance, regrettably, he failed to fulfill those responsibilities."
News of the World said the picture was taken during a November house party while Phelps was visiting the University of South Carolina. During that trip, he attended one of the school's football games and received a big ovation when introduced to the crowd.
While the newspaper did not specifically allege that Phelps was smoking pot, it did say the water pipe is generally used for that purpose and anonymously quoted a partygoer who said the Olympic champion was "out of control from the moment he got there." Phelps and his advisers did not dispute the authenticity of the picture.
The party occurred nearly three months after the Olympics while Phelps was taking a long break from training, and his actions should have no impact on the eight golds he won at Beijing. He has never tested positive for banned substances, and this case doesn't fall under any doping rules.
Phelps' main sanctions most likely will be financial - perhaps from embarrassed sponsors who could reconsider their dealings with a swimmer who hopes to earn $100 million in endorsements.
Phelps was in Tampa, Fla., during Super Bowl week to make promotional appearances on behalf of a sponsor. But he left the city before Sunday's game between the Pittsburgh Steelers and Arizona Cardinals, abandoning his original plan to be at Raymond James Stadium.
USA Swimming said its Olympic champions are "looked up to by people of all ages, especially young athletes who have their own aspirations and dreams."
"That said," the governing body added in a statement, "we realize that none among us is perfect. We hope that Michael can learn from this incident and move forward in a positive way."