With the exception of the French Open, Murray will be among the favourites for every Grand Slam over the next few years. He loves playing at the US Open and has a great chance to defend his title at Flushing Meadows in a couple of months time.
McEnroe won seven Grand Slam titles during his career and he believes that Murray could come close to emulating him with Roger Federer on the wane, and Rafael Nadal's knees remaining a perennial concern.
"He's got a two, three, four-year period - a couple of years especially - where he's going to be tough to beat, really tough," he told BBC Sport.
"He sometimes says hard courts are his favourite surface - maybe he'll beg to differ now - but he has a chance to be number one in the world this year if things go well for him. He could win the US Open to have two majors this year; no-one else would have that."
Murray and Djokovic have now contested three Grand Slam finals over the past twelve months and their rivalry looks set to become the most intriguing in men's tennis but after monitoring the past fortnight's action McEnroe is hopeful that there could be a few more players stepping up to challenge the top two.
"[Maybe] someone else is going to come along," he added.
"Jerzy Janowicz showed up and [maybe] there's going to be another guy that's going to cause problems. [Juan Martin] Del Potro was fantastic. We'll see."