It's another injury for Plouffe, who missed 13 games in April with a right intercostal strain before missing 32 games with a fractured left rib from early July.
"I think it's a similar area, but not sure if it's the same exact spot," Molitor said. "We're looking at the oblique. I had heard he had felt a little something [in his previous at-bat]. He went into the cage and took some swings, and thought he was good to go. But the cut he took finished his night."
Plouffe went 2-for-3 with a solo homer. He's hitting .260/.303/.420 with 12 homers and 47 RBIs in 84 games.
Rhett Bollinger has covered the Twins for MLB.com since 2011. Read his blog, Bollinger Beat, follow him on Twitter @RhettBollinger and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.