Soto, the 2008 National League Rookie of the Year, had to come out of Tuesday's game in the sixth inning because of the discomfort in his shoulder. The Cubs head to Milwaukee after Wednesday's game against the Houston Astros, and Soto was to see orthopedist Dr. Stephen Gryzlo on Thursday.
"According to [athletic trainer Mark] O'Neal, it's not serious, and [Soto] should be OK in a few days," Cubs manager Lou Piniella said Wednesday. "We'll hold him out tonight, and we'll hold him out tomorrow. We'll see how he is on Friday. After today, he'll be day-to-day."
Soto said he had a similar problem a few years ago in the Minor Leagues. At that time, he woke up one morning and couldn't lift his arm. Four days later, he was back behind the plate and the discomfort went away. He said the problem now is not "close" to that.
"From talking to him, he's had this before when he was down in Triple-A Iowa and it lasted a week or so," Piniella said. "Let's hope it doesn't [last long]. If it's going to be that long, I'm not so sure we can stay with one catcher."
Koyie Hill started on Wednesday. Soto can't throw but could be behind the plate if needed in an emergency. Piniella wasn't sure who the Cubs' third catcher would be and planned on talking to the coaches and players prior to Wednesday's game.
Aaron Miles assumed it was him.
"Last year, [Yadier Molina] got banged up and I was going to be the guy who came in," said Miles, who was on the Cardinals then. "I have experience doing it, not in my professional career, but I did it a lot as a young kid and I did it in high school, and I had some pretty good pitchers. I've warmed up pitchers a lot."
He has a little more experience than shortstop Ryan Theriot, who was warming up Aaron Heilman in the ninth inning on Tuesday.
"I'm capable of doing it," said Miles, who has played seven positions in the big leagues, including pitcher. "If they need me, I won't hesitate to get back there."
But it's not his first choice.
"I don't think it is for anybody who doesn't have any professional experience in game situations back there," Miles said, "but when you look at the choices and you're in that position, I'll do it. I have experience doing it. If I get called upon, I'll tackle it."
More players made it clear Wednesday they would prefer not to put on a mask or shin guards. Micah Hoffpauir hasn't caught since he was 12, playing Little League. Mike Fontenot recalled catching Ryan O'Malley in the Minor Leagues and accidentally having his thumb bend backwards while catching a pitch. That was enough for him.
Soto could be back by the weekend. The Cubs will know more after the young catcher is evaluated. The team does not have much depth in the Minor League system if Soto is sidelined for an extended period.
The Cubs began this season without a single player on the disabled list for the first time in 23 years. They'd like to keep that stretch going beyond the first week of the season.
"I woke up feeling a little bit better," Soto said. "I'm taking it easy for right now and see what the trainers do, and I'll follow their instructions."
That means lots of heat packs and ice packs and exercises.
"It's just a matter of staying on top of it, and hopefully it'll go away in a couple days," Soto said. "I want to be out there to play 162. It's just a little setback."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.