Morneau, the 2006 American League Most Valuable Player, was leading the Twins with 30 home runs and 100 RBI, but has been hobbled by injuries recently. He was hitting .274, but had only batted .173 (21 for 121, with 4 home runs and 17 runs batted in) since Aug. 1.
"You can play through the pain if you feel like you're helping the team or doing something to help your teammates out and win ballgames," Morneau said. "When you go out there and you don't feel like you're helping anyone, that's when you kind of take a step back and go, 'All right, maybe I'm doing more harm than good.' "
Morneau took himself out of the lineup Friday after experiencing lower back pain following a slide headfirst slide into first base. The stress fracture turned up on the L5 vertebra during an MRI Monday.
"We took a look at it and found out there was something there," he told the St. Paul (Minn.) Pioneer Press.
Manager Ron Gardenhire said right fielder Jason Cuddyer will play first base most of the time while Morneau sits out to heal. He will not need surgery.
The Twins trail AL Central-leading Detroit by 5 1/2 games and have seven games remaining against the division leaders.