Initial reports in Spanish indicated that he injured his right pectoral muscle, but Twins general manager Terry Ryan said he hasn't heard anything from the club's doctors yet. Guerra was set to be checked out by the club's medical staff on Friday.
"I don't want to give the wrong information," Ryan said, "but it's not a secret. He's back home and has some discomfort."
Guerra, who was acquired by the Twins in 2008 in the trade that sent Johan Santana to the Mets, had a 4.11 ERA with 71 strikeouts in 70 innings split between Double-A New Britain and Triple-A Rochester last season.
Guerra's not on the club's 40-man roster, as he was expected to start the season at Triple-A Rochester.
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.