Saturday is the 15th anniversary of Randy Johnson’s perfect game. Former D-backs manager Bob Brenly sat down with MLB.com recently to describe for the first time a conversation he had that night with bench coach Robin Yount.

PHOENIX -- It was the start of the seventh or eighth inning of the game between the D-backs and Braves in Atlanta on May 18, 2004, and Arizona left-hander Randy Johnson was dealing.

The Big Unit had a perfect game going, but a tenuous 2-0 lead when manager Bob Brenly turned to his bench coach Robin Yount.

“As a manager you’re constantly trying to look ahead and prepare for what could happen,” Brenly said. “I mentioned to Robin that his pitch count was rising, and while pitch count wasn’t an issue with R.J., we’ve all seen it where a pitcher gives up a hit and then the game spins out of control. I wondered if we should get someone warming up at some point just in case.”

Johnson racked up plenty of "high pitch count games" during his career. He had thrown 160 in eight innings against the Rangers in 1992, but he was 40 years old in 2004 and 125 was his high that season.

Johnson was at 91 pitches through seven innings and had thrown 103 by the time the eighth ended on his way to a 117-pitch effort.

“If he sees someone warming up, he’s going to be pissed,” Yount told Brenly.

And the last thing the manager wanted to do at that point was disrupt the rhythm Johnson had going on that historic night.

“I mean we even briefly discussed whether we could have someone warm up in the tunnel behind the visiting bullpen or in the batting cage behind the dugout,” Brenly said. “Somewhere R.J. wouldn’t see. We weren’t thinking of taking him out when he had a perfect game. It was about having someone ready in case he lost it.”

But before they could discuss it further, Johnson mowed the Braves down 1-2-3, so they dropped it.

“We did have some relievers stretching in the back of the bullpen,” Brenly said. “But that was it. We weren’t going to do anything to mess with the game. We didn’t even take left fielder Luis Gonzalez out for defensive purposes, which we sometimes did in close games.”

Here are a few other details from Johnson’s performance:

• At age 40, Johnson is easily the oldest pitcher to throw a perfect game. No other pitcher has thrown one past age 37.

• Johnson’s Game Score of 100 is the third best in a perfect game. His is one of 16 nine-inning outings overall to reach triple digits, going back to 1908.

• Johnson is one of three pitchers to strike out at least 13 batters in a perfecto, along with Sandy Koufax (14 in 1965) and Matt Cain (14 in 2012).

• He is one of seven pitchers to throw a perfect game on the road, doing it in Atlanta -- against a Braves team that won 96 games and a division title.

• The key to the perfect game was that Johnson managed to strike out Chipper Jones in all three of their matchups. Jones, a fellow Hall of Famer, was a thorn in Johnson’s side throughout their careers, with a line of .349/.404/.791 in 47 plate appearances. Nobody hit more homers against the Big Unit than Jones (six), nobody had a higher OPS (1.195) among those with at least 40 PA.