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"If Usain was to be really focused and committed on cleaning up his technique he could probably run 9.4 seconds but he would have to do some major training and adjustments in the way that he runs," Johnson said in an interview with Laureus.com.
"I think he can do whatever he wants to do. If he gets to the starting line healthy, at his best, everyone else at their best, he wins every time ... he's that good."
Johnson, who holds the world and Olympic records over 400 metres, was impressed by Yohan Blake's two victories over Bolt in last weekend's Jamaican trials and believes the 22-year-old is another gold-medal prospect at the London Games that start on July 27.
"Yohan Blake showed he will take advantage when Bolt does not perform at his best," the American said.
"Bolt now has a legitimate challenger and will need to be at his best in London to defend his Olympic title."
Johnson also called for the punishment for drug cheats to be increased from two to four years.
He and his relay partners gave back their gold medals won at the 2000 Sydney Olympics following 4x400 team mate Antonio Pettigrew's admission in 2008 that he had taken banned substances erythropoietin and human growth hormone.
"You have people continuing to use performance enhancing drugs because you could be banned for two years and you could be back for the next Olympics," said the 44-year-old Johnson.
"That's just not enough of a punishment specifically in sports like athletics where the Olympics is really what it's all about.
"A lifetime ban? I think everyone should be given a second chance. You have some people who will make a mistake and I think they should be punished and I think that four years would be a more appropriate ban," Johnson added.
