With one sure swing, McCann gave the Braves their first lead of the series with a shot that woke up the Braves as well as the crowd of 46,181.
From that moment on, the Braves were in control and Clemens and the Astros were in an uphill fight.
"When Brian hit that home run, it changed the whole game," said Atlanta starter John Smoltz, who pitched seven strong innings to earn his Major League-record 15th postseason victory. "That home run for me was pretty much the adrenalin boost I needed."
"That home run is what killed us," Clemens said.
The situation was this: Trailing, 1-0, with two outs in the second, the Braves had runners on first and third and McCann making the first postseason plate appearance of his career.
Clemens threw two consecutive balls to the 21-year-old. With Smoltz on deck and a base open, the prudent thing to do would be to pitch around McCann or even walk him. Most pitchers might have done so.
Then again, this is The Rocket we're talking about.
This is man St. Louis manager Tony La Russa calls one of the greatest competitors in baseball history. This is a seven-time Cy Young Award winner against a baby-faced catcher with 59 games on his resume who had never even taken a swing in a postseason game. Clemens made his Major League debut when McCann was about 12 weeks old.
Under the circumstances, leaving the decision of whether to issue the intentional pass up to Clemens was of course understandable.
"In hindsight, it might seem like a bad decision," Atlanta third baseman Chipper Jones said. "To be honest with you, I think they might have thought they would get McCann out."
And yet walking McCann intentionally probably wasn't necessary either.
Clemens' split-fingered fastball has become an out pitch for the right-hander over the last two seasons and might have done the job this time. Once he was ahead 2-0 in the count, the rookie was sitting dead red on the fastball. Even if Clemens had thrown a splitter or two out of the strike zone, maybe the kid chases them.
Maybe not, but so what? In that case, McCann walks to load the bases with Smoltz due up.
Clemens, however, went with his fastball on 2-0 to McCann. With catcher Brad Ausmus setting up to take the pitch away, the ball was more on the plate than off it and McCann lashed it over the right-field wall.
"It was a fastball that was up and caught more of the plate than I wanted it to," Clemens said. "It was just a matter of my location being off a little bit."
Clemens also had trouble with his slider and the muddy conditions made gripping offspeed pitches problematic. So Clemens went with his fastball. The velocity was fine, the the location was off and the end result was a pitch that Clemens called "very hittable."
McCann hit it very hard.
"I told myself I was going to go up there hacking trying to get a pitch out over the plate," McCann said. "I was just trying to put the bat on the ball. I mean, that's Roger Clemens out there, no way I was thinking home run."
That thought wouldn't cross McCann's mind until after he cleared first base and dared a peek to right. Not that he needed to look. The crowd had already told everyone in Atlanta the ball was long gone.
"I know I won't ever forget it," McCann said.
"It's going to be in his scrapbook for the rest of his career," Smoltz predicted. "Roger Clemens, whether it's his 'A' game or 'B' game, that's a special one for anybody."
It was also special because it gave the Braves the spark they needed, as well as the lead against an Astros team which will never be mistaken for one of the better come-from-behind teams around the league.
The Braves would go on to score two more off Clemens in the third and two more off reliever Chad Qualls in the seventh, but McCann had given Smoltz all the offense he would need.
The Braves won the game they could not afford to lose.
As for Clemens, he has been so sensational for so long it is stunning on those rare nights when the 43-year-old reminds us that he is still mortal. He has been money in the bank for Houston.
Unfortunately for the Astros, this was just that rare night when another team cashed in.