Posada was batting for Jose Molina when he sent a fly to right off Jensen Lewis with one out in the seventh inning of the Yankees' 7-3 win. Trevor Crowe leaped at the wall, but the ball was deflected by a fan and bounced off the top and then off Crowe's glove before falling into play.
"I was hoping the ball would carry the way it's been carrying," said Posada, who ended up with the 20th homer in the first four games at the new Yankee Stadium. "I didn't think it was gone, though."
The game was delayed about 8½ minutes overall.
The play brought back memories of Derek Jeter's home run against Baltimore in the 1996 playoffs, when young fan Jeffrey Maier reached over the wall in nearly the same spot at the old Yankee Stadium to catch a drive above Orioles right-fielder Tony Tarasco.
Video replay wasn't available back then - it's in its first full season of use by Major League Baseball after getting a trial run late last season.
Posada hesitated near first base because couldn't see the ball until it hit the warning track, then kept running. He didn't know it was a home run until he was past second base and approaching Cody Ransom, who had held up at third. That's when Posada saw third base umpire Mike Estabrook signal home run, giving the Yankees a 4-3 lead.
Crowe ran toward the infield indicating a fan had interfered, and Indians manager Eric Wedge came out to dispute the call.
"I didn't take into account that I'd have to get above the fan to get the ball," Crowe said. "I watched the replay and it looked like his glove was outstretched on top of mine. It all happened so fast."
The umpires convened near the mound before going to the video room through the visitors' dugout. They returned and summoned both managers. Girardi was called for first, causing the crowd of 43,068 to boo after celebrating minutes earlier.
"When he came out with his explanation, as soon as he started I felt pretty good," said Girardi, who acknowledged a moment of nervousness.
"I thought it was a home run, I did," Girardi said. "I know it was very close and a lot of times you could be wrong with the naked eye. I actually thought it was a home run."
While the umpires discussed it on the field, Girardi ran the few steps into the video room just off the dugout - a perk of the new stadium - to give it his own review.
"I had looked at it, and looked at it and looked at it, and had other guys look at it," Girardi said. "And we all came up with the same answer, so we figured it wouldn't be overturned."
Wedge saw things differently, but didn't protest much after the umpires gave their ruling.
"I thought it never got on top of the wall where the fan was. My argument was the fan and the glove came together, but the replay, they said it was beyond the fence," Wedge said. "They had limited views. They did the best they could."
Umpire crew chief Jerry Crawford declined comment after the game.