After over two years without a major victory amid a slump in form and persistent elbow injury, a rejuvenated Djokovic returned to the peak of his powers to win Wimbledon in 2018.
The Serbian has since triumphed at the US Open and Australian Open to take his grand slam haul to 15, two shy of Nadal and five behind Federer.
Djokovic will hold all four majors simultaneously for the second time in his career if he wins the French Open in June, though Becker considers Nadal favourite to triumph at Roland Garros for the 12th time in his career.
However, the German believes his former pupil can still amass a record-breaking number of grand slam titles.
"A lot can happen in a year either way. I mean, you've seen it. Yes, on paper, he's 31, he's won 15 majors, yes, there's a possibility he can reach 20," Becker told Omnisport at the Laureus World Sports Awards 2019.
"But who says that Roger isn't going to win another one. I always liked his chances at Wimbledon on the grass. Who says that Nadal isn't competing for it? He was in the final in Melbourne. He's certainly the favourite in my book for the French Open, and then he has 18, there's only two more for the French.
"So, it's a good problem to have because ultimately, you want to be the most successful of all time and what a challenge. What a moment in all three players' lives."
Expanding on whether Djokovic can depose Nadal at the French Open, Becker said: "First of all, it's amazing that he's in a position to do so. Very few players in the history of tennis can say that.
"Now, playing Nadal at the French is the most difficult tennis match you're ever going to face. Novak has done it a few times, he's beaten him one time on the year when he won the Grand Slam, and that's going to be the match.
"The way the rankings are, I think it's one and two, the earliest it can be is in the semi-final - I mean, what a match! Who wouldn't want to watch that match? History will be re-written, but there's a lot of tennis to be played until then.
"Players are going to the hard courts in America and on the clay in Europe so hopefully nobody gets injured, they keep the momentum, they play enough, but not too much. So the French Open should be an amazing high."