Having seemingly recovered from his knee injury struggles, McEnroe is confident that Nadal can still win a handful of major crowns before he retires.
"If he stays healthy, he can easily win four or five more Slams," McEnroe told BBC Sport. "No question about it. To me, he's better than ever.
"It's amazing how badly he wants it. I've never seen anything like it. Let's hope he stays healthy because it will be great for our sport."
Federer, who has won 17 career titles, has not won another major since 2010.