Nadal has not played since his hip injury at the Australian Open and the 14-time winner at Roland Garros said it could be months before he returns to action as he prepares to end his career in 2024.
"Honestly it was just so one-sided the way Rafa was able to dominate that all you can talk about is Rafa," McEnroe told Eurosport.
"Novak has as many Grand Slams - 22 - but you don't put him in the same breath there (at Roland Garros) as Rafa. He set a bar so high that it's impossible to imagine anyone even get close to it.
"So from that standpoint alone, it's much more open ... The ideal would have been, if Rafa was still playing at a very high level and someone was good enough to beat him when he's playing great. That's what people really wanted to see."
McEnroe said Djokovic has a great chance to win a calendar slam this year but he added youngsters like world number one Alcaraz and Holger Rune (20) can make the step up.
"I would say Alcaraz would be the slight favourite to win this," McEnroe said.
"Alcaraz has a lot of pressure now because he's just such an unbelievable breath of fresh air. He's so electric, the way he plays, he's got such a great personality that people want him to win.
"But then you got to back it up. He's 20 years old, but he just seems like such a great ambassador for our sport that you're sort of pulling for him."
