Initially, Morrison held up for a double, as no home run ruling was called.
"I thought it was out," Morrison said.
Morrison said he was scared when he saw third-base coach Joe Espada pointing to return to second base.
"I was like, 'I might get thrown out here,'" Morrison said. "I thought I hit it out. ... I knew I got it well, but at this park, you never know, with the high fence and everything else."
Marlins manager Ozzie Guillen asked crew chief Jerry Layne for a review. The umpires huddled, and headed off the field to check the review.
After a short delay, the umpires returned to the field and determined the ball was indeed a home run. The run became the game-winner for Miami.
From the dugout, Guillen saw the ball clear the wall, and he was confident the double would be overturned.
"Yeah, I know," Guillen said. "As soon as he hit it, I knew that ball hit the back of the rail. Thank God, we have instant replay, because now it was going to be a double. That helped a lot. It also helps the umpires."
That's quicker than arguing for three hours.
Morrison's home run was his eighth off the season.
The Marlins hit back-to-back homers in the inning. John Buck went deep off Fernando Salas. The Cardinals made a pitching change, and used the left-handed Freeman for Morrison.
It was the second review ever at Marlins Park.
On June 7, Giancarlo Stanton had a home run declared by replay on a drive also to right field.