"If he learns just a little bit (about sprinting), there is no telling what that boy can do," Greene, now retired, told Reuters in an interview. "He's mind-blowing."

Bolt, 22, lowered the 100m world record to 9.69 seconds and the 200 mark to 19.30 seconds in stunning runs at last August's Beijing Olympics. He also helped Jamaica claim the world 4x100m relay record.

"I don't know what to think about him," said Greene, who won the Olympic 100m gold at the 2000 Sydney Games.

"He's doing things that no one in history has ever done. He's a great talent."

Maturity and experience will make Bolt even faster, the 34-year-old Greene said.

"He's still a boy (physically)," the American said. "He has not come into a man of a body yet."

Bolt, who had run only one professional race at 100 metres prior to 2008, must also develop better sprinting technique, Greene said.

"He doesn't know what he is doing so he has nothing but improvement (ahead of him)," said Greene, a three-time world champion at 100 metres and the first man to win 100 and 200 titles at the same world championship.

"If he learns a little bit of what to do, he is going to get better. If he runs the race all the way through, he is going to get better."

Just how fast is a matter of speculation, especially after Bolt shattered the 100m record in Beijing while looking around and pumping his arms 20 metres from the finish.