Johnson compared Bolt's style to that of Tyson Gay or Asafa Powell stating the reigning 100m and 200m Olympic champion is not a very clean sprinter from a bio-mechanical standpoint based on an analysis done at the Michael Johnson Performance Center in Dallas.
Johnson acknowledged that Yohan Blake, who won the 100m in the Jamaica Olympic Trials in Kingston last week, could prove to be a challenge if Bolt does not perform at his best this summer.
However, the 44-year-old still feels Bolt - who will skip this month's Diamond League meet in Monaco with a "slight problem" - is a firm favourite to retain his 100m, 200m and 4x100m titles in London.
"Yohan Blake showed he will take advantage when Bolt does not perform at his best. Bolt now has a legitimate challenger and will need to be at his best in London to defend his Olympic title," Johnson said to Laureus.com.
"But I think Usain can do whatever he wants to do! He's got that type of talent and he wants to win three gold medals again.
"I would say if he gets to the starting line healthy, at his best, everyone else at their best, he wins every time, I mean he's just that good.
"He could break the world record again, he could run 9.5 secs, lower. Ultimately if he were to really be focused and committed on cleaning up his technique, he could probably run 9.4 secs, but he would have to do some major training and adjustments in the way that he runs."
Johnson, who gave up a gold medal in the 4x400m relay in Sydney after team-mate Antonio Pettigrew admitted he had used performance enhancing drugs, stated the two-year ban on athletes in the case of doping was not a strong deterrent.
"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," he said.
"And that's just not enough of a punishment, specifically in sports like athletics where the Olympics is really what it's all about."