Johnson won the 200m title at the Atlanta Games in a then-world record time of 19.32 seconds, which stood for 12 years until Bolt ran 19.30secs at the 2008 Beijing Olympics.
The Jamaican bettered his own mark a year later at the World Championships in Berlin, clocking 19.19secs, and Johnson believes the 27-year-old could make history and dip under the 19-second barrier.
"I think he could go under 19 seconds but then beyond that I'm not quite sure," Johnson said, speaking at the Laureus Sports Awards.
"Technically, he's not the best. Technically he's a little bit all over the place and that's a race where the longer it goes, the more you need to be really efficient in order to be able to maintain the level of speed that you want.
"And so that's something that, if were to clean up some of those things, I think he could go under 19 seconds."
Bolt has been utterly dominant in the sprint events over the last six years, winning gold in 100m, 200m and 4x100m at every Olympics and World Championships since 2008, with the sole exception coming after a disqualification for a false start in the 100m final in Daegu in 2011.
Johnson feels that Bolt has probably reached the peak of his career, but was hesitant to say that the Jamaican would now experience a decline in form.
"If I had to guess and go out there and say whether or not we've seen the best of him, I would say probably, but you never know with him," he said.
"There's the argument for both. You could say that, as a sprinter gets older you're not going to get faster, probably, you're going to get slower and not faster.
"Then on the other side of things, I don't know if he has done everything that he possibly could to go out there and be the best that he can be."