The Cubs placed Soriano on the 15-day disabled list on Thursday.
"You know this is a costly win," Cubs manager Lou Piniella said after the Cubs' 7-2 win over the Braves. "Alfonso was playing really well."
"It's hard to sit back and enjoy the victory when one of your best players, one of the leaders of your team, goes down. He's proven in the past he's a pretty darn quick healer," Chicago starter Ryan Dempster added. "It's up to the other guys to pick him up."
After he was hit in the second inning, Soriano left the game and went to a hospital for X-rays. He will be in a splint for three weeks, Cubs spokesman Peter Chase said.
Soriano was on the DL earlier this season with a calf injury -- the Cubs were 9-5 during his absence -- and was just beginning to look comfortable on the bases. After a slow start this season, he came off the DL the first time and during one stretch last month hit seven homers in six games.
"He was just starting to run the way we thought he could, playing well in the outfield and hitting the ball with some power," Piniella said. "A bad break for us. Just an unfortunate thing. ... It's a shame, it really is. Things happen in baseball."
After a slow start that led to Cubs fans booing him, Soriano had found his groove recently and helped the Cubs move into the NL Central lead with the best record in the major leagues. He entered the game with a .284 average, a team-high 15 homers and 40 RBIs.
Bennett's inside pitch caught Soriano as he tried to turn away and he immediately sunk to the ground.
Trainer Mark O'Neal and Piniella came out of the dugout. Soriano was on one knee while O'Neal examined him, then walked to the dugout.
"That's terrible. I feel bad for him. But no means was it even close to being intentional," Atlanta catcher Brian McCann said. "We had him 0-2. We were trying to go in with a fastball, hopefully lock him up. Obviously it got away from him."
Bennett was not available for comment.
"He fought hard to get back. He was just getting his legs back and then has to go through this," Cubs first baseman Derrek Lee said. "He's going to be in a cast and that really limits what you are going to do. I'm sure he's frustrated. I just feel for him."
Soriano was not in the clubhouse after the game. The Cubs will use Reed Johnson in left and could call up some players from Triple-A.
"What can we do?" third baseman Aramis Ramirez asked. "We've got to move on. ... It's a big loss. He leads the team in homers, 40 RBIs. You can't replace that."
The Associated Press contributed to this story.