The American former world number one watched Henman's tentative first round win over Spain's Ruben Ramirez Hidalgo on Tuesday and was not overly impressed by the Briton's performance against a man making his debut at Wimbledon.
"Tim Henman may have won his first-round match but it was not a good performance or the level you would expect from him," McEnroe told BBC television after the four-set contest.
"That was surprising to put it mildly, considering how well he played in the French Open."
"It looked like he was overwhelmed by the expectation on him at the start."
But McEnroe said he expected the four-times semi-finalist to raise his standard
"Tim knows he needs to play a lot better than that if he wants to win Wimbledon, but then you have to win matches like that if you are going to win it."
"You would hope that would be the only match where he is going to really struggle."
"Having got that first one out of the way, he will be better prepared and feel more relaxed in the second round."
Another three-times champion, Boris Becker, was impressed by Henman's ability to find a way to win.
"I was shopping in Harrods with my children and saw it on TV when he was 5-2 down in the second set tiebreak," the German said. "At that point Tim played some great shots."
"There was one great volley at set point down and that may be the reason we see Tim in the second week here."