Raducanu upset the odds to become the first qualifier to win a grand slam during the Open era on Saturday, beating fellow teenager Leylah Fernandez in the final at Flushing Meadows.
Her breathtaking performances in New York came off the back of a promising debut at Wimbledon just two months before, where she reached the fourth round before withdrawing with "breathing difficulties."
"Totally unexpected. Never thought it would happen in my lifetime to see an 18 and 19-year-old do it," McEnroe told CNN's Christiane Amanpour, speaking of the all-teen final at the US Open.
"I think the women's game is more open than it's ever been so that's a sign that a lot more players think they have a chance.
"I can't imagine anyone in her [Raducanu] camp expected this. To win the US Open without losing a set, that's crazy. She's a tremendous athlete, she seems like a great kid."
Following Raducanu's withdrawal from Wimbledon, McEnroe was criticized for saying the situation had become "too much" for her.
At the time, it was not clear what made the youngster stop playing, but she later said the "whole experience caught up with me" and that she began to "breathe heavily and felt dizzy" after a number of "super intense rallies."
McEnroe received backlash from people saying he was questioning the youngster's mental strength but insists he never meant to cause offense and only ever intended to support the breakthrough star.
"I meant exactly what I said," he added. "I tried to relate it in a small way to my experience when I first went to Wimbledon also at 18 and managed to qualify like Emma did.
"I played Jimmy Connors, I hadn't been on Centre Court. I remember my leg shaking and being totally overwhelmed by the experience and almost happy I didn't win.
"There's a lot of great upsides, but there's also pressure you put on yourself and expectations that others put on you.
"Compared to a lot of other things I've said in the past, I mean that was to me as vanilla as it comes. I was very supportive of her, I thought, at the time."
The seven-time grand slam singles winner -- who was renowned for his mid-match antics -- says he hopes Raducanu can deal with the newfound attention on her career.
The teenager was barely known outside of tennis before this year but is now one of the biggest stars in the game.
"I don't think you could possibly do it any better than she did it [at the U.S. Open]," McEnroe added. "That's insane that she's been able to do this."
"Of course, pressure comes with that, expectation comes with that. I'm sure she expects to win a lot more moving forward."