Djokovic, 34, claimed a record-equalling 20th men’s title by beating Italy’s Matteo Berrettini in Sunday’s Wimbledon final, matching the record held by Roger Federer and Rafael Nadal.
Only Margaret Court, Serena Williams and Steffi Graf have more, but McEnroe expects Djokovic to surpass them too.
“Djokovic is playing better than he has ever played,” he told BBC Sport.
“I think he will probably win at least four or five more, obviously depending on staying healthy.”
After triumphing in the Australian Open and French Open too, Serbia’s Djokovic has now won all three Grand Slam titles this year.
With the Olympics and US Open coming up, he could emulate Graf’s 1988 ‘Golden Slam’, though says he is “50/50” about playing in Tokyo.
“Djokovic has put himself so far out in front of everyone in terms of his ability to embrace what he is doing – in terms of creating history – and being able to execute under a lot of stress,” said McEnroe, who won seven Grand Slam singles titles.
“You’re trying to break the all-time records – there is a lot of pressure. He’s able to play his best tennis at this point.
“You expect that to go on for another couple of years, unless someone steps up and realises how great they are.”