Duncan flew to St. Louis on Monday morning to meet with team
doctors. Manager Tony La Russa wasn't sure how long Duncan would be out and called his return this season "questionable."
Duncan is batting .259 with 21 home runs and 70 RBI.
"He's a sore young man," La Russa said. "I don't know. I mean, he's been sore for a couple of months."
Duncan left the game during the Cardinals' 9-8 loss in Arizona on Saturday after he aggravated his injury running the bases in the fifth inning.