Soto underwent an MRI exam Thursday in Chicago, and the Cubs said it turned up nothing more than inflammation and irritation.
"I would say tomorrow, probably not," manager Lou Piniella said, referring to Saturday night's game. "Sunday, I'll waver a little bit, but I wouldn't bet on it."
Soto, last year's Rookie of the Year in the NL, is not taking batting practice. Instead, he is working to exercise his shoulder and do his conditioning work. Soto has a tendency to put on weight, so the Cubs want to make sure he stays active.
One thing the Cubs will not do is let themselves be short-handed for too long. Koyie Hill is the only backup catcher they have, with infielder Aaron Miles a possibility in an emergency.
If Soto would have to go on the disabled list, veteran Mark Johnson is catching at Class AAA Iowa. Piniella said the situation can't be allowed to go on for too much longer.
BREWERS 4, CUBS 3: RHP Rich Harden struck out 10 in six innings, but closer Kevin Gregg allowed two runs in the ninth as the Brewers erased a 3-2 deficit. Milton Bradley hit a solo homer, and Hill hit a two-run blast. Cubs relievers walked five, including two by Gregg in the ninth.