The 34-year-old Serb beat Italian seventh seed Matteo Berrettini 6-7(4), 6-4, 6-4, 6-3 in the final, and McEnroe, a seven-time major winner, told the BBC that he was "playing better than he's ever played", Reuters reported.
"I think he'll probably win at least four or five more ... depending on staying healthy," added the American.
"He's put himself so far out in front of everyone in terms of his ability to embrace what he's doing – in terms of creating history - and being able to execute under a lot of stress.
"You're trying to break the all-time records – there's 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."
Djokovic's coach and former Wimbledon champion Goran Ivanisevic said that the debate over the greatest player of all time could be settled if the Serb wins the US Open and completes the calendar Grand Slam.
Rod Laver was the last man to win all four majors in a single year in 1969.
"He's the only guy who can win four in a row in the same year," Ivanisevic told British media. "If he wins the US Open, (the debate) is over."