In the fourth inning of St. Louis' regular-season opener against Colorado, Duncan singled to center field. He was then thrown out at second base on a "run-and-hit" play as Albert Pujols struck out and Duncan was tagged out sliding into second.
"Rounding the base, I felt a little sore," Duncan said. "And then I ran on the 3-2 pitch and felt something. I played the rest of the game fine. It was just sore. I went to get it iced, and they think I should take it easy today. Be precautionary since it's early. We don't want it to turn into something, particularly with the cold weather."
The play was already costly on the field, as the double play short-circuited what might have been a promising rally for the Redbirds.
"That was a big play in the game," manager Tony La Russa said. "If you're going to put [that play] on with anybody -- if you're going to start a runner with 3-2, [Pujols] doesn't strike out very often."
For Wednesday's game, Ryan Ludwick took Duncan's place in the batting order against Rockies right-hander Aaron Cook. Skip Schumaker moved from right field to left field, and Ludwick started in right. Typically against a right-handed pitcher, Duncan will be the left fielder and Schumaker will be in right.
It has been a frustrating start to the year for Duncan, who began Spring Training in a slump then missed several days with a sore back. He started to find his stroke at the end of Grapefruit League play, finishing the spring on an 8-for-28 stretch with six walks, and he was 1-for-3 with a walk on Tuesday night.
Now he's sidelined again, and though the injury is believed minor, there's no certainty.
"I'm going to make sure it's well before I run him back out there," said La Russa. "I'm not exactly sure how long he'll be off or how long you've got to be careful with him."
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.