Santana said he first felt pain in his back when he reached for a ground ball during batting practice Friday.
"I stood down for a little bit, and it was hard to stand up, and I tried to run, and it was hard to run, so I knew something was going on," Santana said.
Santana said his back felt better Saturday than it did Friday, but it still hurt him to walk. The right-hander will not be placed on the disabled list, and the Twins hope he is able to make his next start.
Duffey made 10 starts for the Twins last year, going 5-1 with a 3.10 ERA. He has allowed only three earned runs in 15 2/3 innings at Triple-A this season.
• After playing center field with Class A Advanced Fort Myers on Friday, utilityman Danny Santana (right hamstring) was scheduled to play right field Saturday and third base Sunday, in hopes of rejoining the Twins when he is eligible to come off the disabled list Monday.
• Antony said third baseman Trevor Plouffe (right intercostal strain) and closer Glen Perkins (left shoulder strain) are both exercising, but neither has resumed baseball activities as they continue their rehab.
Alex Putterman is a reporter for MLB.com based in Washington. This story was not subject to the approval of Major League Baseball or its clubs.