But the big-serving Texan hasn't played since suffering a foot injury in his final loss to Roger Federer at the Miami Open in March and none of his countrymen are at a level where they can threaten the best, McEnroe said in a call with reporters.
"I think that the odds are pretty low," the American seven-time major champion said when asked if compatriots might challenge.
"Isner has been out ... so he's sort of an unknown quantity.
"And the other guys to me, whether it's (Reilly) Opelka, who is like a younger version (of Isner) or (Frances) Tiafoe, who has been struggling -- it doesn't look to me like there are guys ready to go and make that move."
McEnroe said the situation would improve as up-and-comers like 21-year-old Taylor Fritz gained experience but for now the stranglehold that veterans Federer, Rafa Nadal and Novak Djokovic have on the sport would likely continue.
THE LURE OF FOOTBALL, BASKETBALL
Despite being a country of nearly 330 million people, American men have not been much of a factor in tennis since the retirements of 14-times Grand Slam champion Pete Sampras in 2003 and eight-times major winner Andre Agassi in 2006.
Sampras was the last American man to win Wimbledon in 2000, his seventh.
One reason is that the best male athletes from the U.S. tend to gravitate toward more accessible sports.
"We've been seeing this for years or decades, but our best athletes seem to be playing football and basketball more. Soccer has grown as well, so you need to go after athletes," he said.
"Tennis, if anything, is more expensive than it was before. So the accessibility is a big factor."
He pointed to Tiafoe, the son of immigrants from Sierra Leone, as one to watch.
"We have Tiafoe who is a great athlete but he's still learning," he said of the 21-year-old who learned the sport at a facility in Maryland where his father was head of maintenance.
Attracting more natural athletes like Tiafoe to the sport will be key if American men are going to hoist trophies at majors or compete for the world's top ranking.
"There are going to be guys that are dangerous but at the moment, the more athletes in the game because of the quickness of the game, the better chance we'll have of making that breakthrough," McEnroe said.
(Editing by Ian Ransom)
