Busch followed with a tweet that said he felt "very confident with my info".
"Guess we'll see," he said.
Phelps has said repeatedly his swimming career is over, having won 18 gold medals and 22 medals overall, more than any other Olympian.
He retired at the age of 27 immediately after last year's London Olympics.
"I wanted to leave that way," he said. "I'm sure I could come back in another four years, but why? I've done everything I wanted to do. There's no point in me coming back.
"Everybody is like, 'You're going to come back.' And I'm like, 'No, I'm not.' I've done everything I wanted to do in this sport. I don't know a lot of people who can say that."
Since London, Phelps has turned his attention to golf, filming a show with famed instructor Hank Haney and signing an endorsement deal with Ping.
Last week, he participated in an event prior to the Tour Championship at the TPC Sawgrass course in Florida.
- AP