Bolt has dominated the 100m and 200m at major championships since winning both in world record times at the 2008 Beijing Olympics, only a false start in the short sprint final at the 2011 world championships preventing him from a clean sweep.
Joint problems have hampered the 29-year-old's season, however, and he returns to the Bird's Nest stadium for the 100m heats on Saturday with best times of 9.87 and 20.13 seconds for each sprint.
Gatlin, although he has yet to go head-to-head with Bolt this year and has beaten the Jamaican only once in seven attempts over the 100m, has not been beaten in either sprint for two years.
Tokyo 2020
When are the 200m sprint finals at the Tokyo Olympics? What are the current world record times?
02/08/2021 AT 10:15
The 33-year-old owns the best times of the year in both events with a personal best of 9.74 in 100m and 19.57 in the 200m.
"Right now, if I have to go by what's happening right now you would say Justin is going to win," Greene said. "But the biggest problem is you never know what Usain is going to come out with because, you have to remember, he's been where nobody has been before.
World Championships LIVE on British Eurosport: TV schedule
"So who knows if he's going to come back with something like that? It's very well possible so you can never count him out. Bolt don't ever run a lot. It doesn't matter if he's run four races or 12 races, you're gonna get a race."
Watch this event on Eurosport Player
The Beijing sprint showdowns between Bolt, who has never failed a drugs test, and Gatlin, who has served two suspensions for using banned substances, have been billed as battles for the soul of the sport.
The last two rounds of the 100m both take place on Sunday evening and Greene said the semi-finals would be a good guide to what would happen later in the evening.
"The semis is going to really tell a lot," he said. "It's going to show you if Bolt is really ready and then you'll be able to make your decision for the final."
[ALL YOU NEED TO KNOW ABOUT BEIJING 2015: World Athletics Championships 2015: Dates, schedule, live TV coverage, tickets, how to watch]
The 200 metres final takes place next Thursday and Greene felt his compatriot had an even better chance of becoming the first major championship gold medallist apart from Bolt since Tyson Gay won the 2007 world title in Osaka.
"I got to see the 100 first but in the 200, I would be more inclined to pick Justin," he said. "Just because it's a harder race and you've got to be really fit and we know how fit Justin has been.
"So that will be easier to pick but you never know what Usain's going to come back with."
Bolt has, of course, run much faster than Gatlin's season-leading marks and holds the world record for both events with the 9.58 and 19.19 he ran to win gold at the Berlin world championships in 2009.
The Jamaican knows he is not in the kind of shape to run those sort of times in Beijing but said on Thursday that Gatlin would have to run the fastest race of his life to take his 100m title.
"I used to say when I was running 'I'm gonna go some place you can't'," Greene, a former 100m world record holder, added. "That's just what Usain is saying, he knows he can run 9.6, he's done it before and if you can't go there, you don't have a shot.
"But let's just see if he can go there also."
Tokyo 2020
Marcell Jacobs the obvious successor to Bolt's sprint king crown - Rutherford
01/08/2021 AT 17:35
Tokyo 2020
100m world record time: What is Usain Bolt's fastest ever time? When is the men's 100m final?
30/07/2021 AT 15:17