The Cubs did not officially place Soto on the disabled list Friday because they were waiting to make a corresponding roster move.
This will be the 13th time this season the Cubs will place a player on the disabled list. Soto would be the 12th player; Aaron Miles has been out twice because of arm injuries.
"It's a tough break for our ballclub," Cubs manager Lou Piniella said Friday. "It's not a real bad one, but it's still going to take time to heal."
Soto, the 2008 National League Rookie of the Year, is expected to go to the Cubs' facility in Mesa, Ariz., on Sunday to rehab.
"He's going to be out probably for a month or so," Piniella said, although he added it could be a little less.
Soto was injured during his first round of batting practice on Wednesday.
"I kept hitting the ball to the middle and I felt something grab me," Soto said. "The second round, I went in there again and I couldn't do it anymore. They said it's 'mild to moderate.' The doctors say it's not even close to the worst you can have, so that's a good sign."
Soto, who was batting .230 with eight home runs and 27 RBIs, said he wasn't sure if he could catch at all.
"Every time I have to use my core, it's there," he said of the pain. "We have a huge series, starting today, and I was looking forward to being there for my teammates."
Koyie Hill started Friday and will handle the bulk of the catching duties, but Piniella said they are looking outside of the organization to add another backstop.
Mark Johnson is the only catcher at Triple-A Iowa with any Major League experience, and he's on the disabled list. Steve Clevenger is a converted shortstop, still learning the job, and Chris Robinson, acquired from Detroit in August 2006, is the Iowa team's other catcher.
Jake Fox -- who has caught in the Minor Leagues, including a couple games this year at Iowa and is on the Cubs roster -- will be the backup for now.
"They told me to get ready in case I'm needed," Fox said. "It's always been a dream [to catch in the big leagues]."
On Monday, the Cubs activated Aramis Ramirez, Reed Johnson and Angel Guzman from the disabled list, but the next day had to place Ryan Dempster on the 15-day DL because of a broken big toe. Now, Soto is out.
"You can't complain about it," Piniella said of the rash of injuries. "It's unfortunate, but what are you going to do?"
In another move, the Cubs recalled right-handed pitcher Jeff Stevens from Iowa, and he will help in the bullpen during the three-day, four-game series against the Cardinals.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.