These injuries can take up to a month to recover from, if the strain is significant. The Cubs said the results of the MRI wouldn't be disclosed until before Friday's game against St. Louis.
It is the latest in a long line of injuries for the Cubs. When Aramis Ramirez returned from the disabled list on Monday, it was only the second time all season that manager Lou Piniella had his Opening Day lineup intact.
Ryan Dempster went down with a broken toe on his right foot Tuesday and is on the disabled list.
The team's options for another catcher at Triple-A Iowa are right-handed hitting Chris Robinson and left-handed hitting Steve Clevenger.
Soto, the reigning NL Rookie of the Year, has struggled all season at the plate, hitting .230 with eight home runs and 27 RBIs.
Bruce Levine covers baseball for ESPNChicago.com.