"You're not going to ask me about Lendl, right?", he asks, tongue firmly in cheek. If ever there is anyone perfectly qualified to discuss what Ivan Lendl can bring to Andy Murray as his coach, then McEnroe is the man.
For the best part of a decade, McEnroe and Lendl snarled at each other over nets across the world, talent and guile against gristle and muscle, pure unbridled aggression against steely determination, a relic of the Cold War.
But it was against McEnroe, at the French Open in 1984, where Lendl had his breakthrough, coming from two sets down to beat the American and win his first grand slam title.
Until then, Lendl had been the nearly man, perhaps the Murray of his time, losing his first four grand slam finals before finally snapping the streak on the clay of Paris.
In a small room at Roland Garros, just yards from the court where Lendl's win had dashed his own French Open dreams, McEnroe said there had always been a question mark about whether Lendl had what it took to win the sport's biggest prizes.
"I think there was an 'if'," he says in his New York drawl, leaning back, arms folded. "He'd got to a stage where he had been knocking on the door but when you don't win the big ones early people start to question everything. Your heart, your fitness, everything, how deep in the well you are willing to dig, how good you are compared to other guys. You have to keep persevering.
"An actor goes to a lot of auditions and doesn't get the part. Jack Nicholson didn't get anything until he was in his 30s. You have to persevere and put yourself in positions and sooner and later you will break through."
As Murray prepares for another assault on Wimbledon, there are obvious parallels between his career to date and that of the man now in his corner as coach.
The Scot has lost his three grand slam finals and as the world No.4 goes into the Championships still searching for his big breakthrough. When he took Lendl on as coach on New Year's Eve, there were some, including McEnroe, who felt that it was a match made if not quite in hell, then certainly not in heaven.
Reaching the semi-finals at the Australian Open and going within five points of beating Novak Djokovic was a great start but since then, his consistency has been found wanting at times.
Lendl was not present for his second-round defeat in Indian Wells and was also missing at the Rome Masters but has been alongside Murray since the French Open and will be there right through Wimbledon.
"Actually this is the first time you can get some kind of gauge of how it's going," McEnroe says. "It was too early in Australia. At first I thought it was crazy, then I thought it was going to work out. Now we'll have to wait and see. If you look at it as a long-term project, if you think they might work together for a few years, you have to give it some time."
In the build-up to the French Open, Murray revealed he had been struggling with a niggling back injury, on and off since the start of the year.
In Paris, he suffered a spasm – unconnected to the original injury, he said – and almost went out in round two, before soldiering on and improving as he reached the quarter-finals. McEnroe, who experienced back problems at various stages in his career, wonders whether Murray's mind, as well as his body, has been affected.
"I had issues with my back and I always wondered how mental or physical that was," he says. "A lot of stress goes to that part of the body – and he's under a lot of stress.
"To me, it's somewhere potentially where Lendl could help him. If you have a coach who's not going to take 'no' for an answer, you could potentially have someone who won't let him – instead of looking for a way out, he'll keep him in there"
"I just remember watching [Roger] Federer play Wimbledon the first year he won it, and he was struggling with his back problem, it looked like there was a chance he was not going to finish – he had that look in his eye.
"And then somehow he found the wherewithal to dig a little deeper. And suddenly the guy goes on and wins the thing and he's like a different player."
On good days, Murray will tell you he is lucky to be born in the same era as Nadal, Djokovic and Federer, that pitting himself against them is the ultimate test. But it is hard not to think that without them around, Murray might well have won at least one slam already and as long as they are still playing, uncertainty remains.
"Is he as good as these guys?" McEnroe asks. "The others are unbelievably good. Murray is a great player. He's bordering on great. But is he as great as those guys? That's quite a different issue. It can go either way. I have been saying this for years but it would be great for the game if Murray can win. I'm sure everyone here is hoping and a lot of tennis fans are hoping that. It's difficult to say but it seems like the gap is widening."
At the age of 53, McEnroe has mellowed, as anyone who listens to his informative and ever-entertaining commentary on BBC TV each summer will attest to. He even says he gets on fine with Lendl but there is one thing he still can't stomach – the widespread reports about how funny the Czech can be.
"I don't know who wrote this article, but a guy that I work with showed me something that said Ivan has come in and brought a sense of humour and that he's a lot more fun than Andy," he says. "I'm like: 'My God, man! What has Andy been doing?' That I really found humorous."
John McEnroe commentates for BBC TV at Wimbledon and hosts the tennis phone-in '6 love 6' on Radio 5 live. Live BBC coverage starts 25 June across TV, radio and online.