"It's still hold on there a little bit," Twins manager Paul Molitor said. "[Santana] did get some extra treatment. But we're not really ready to say when he's going to be slotted back in yet."
Twins general manager Terry Ryan said he's optimistic Santana's injury is only minor, and that he should be able to avoid the 15-day disabled list.
"I don't know if it's another day, two days or four days but we're trying to get through it," Ryan said. "I've never known back spasms to be lingering. But I guess you have to worry about that in any scenario."
Duffey, who was recalled to start in place of Santana on Sunday, gave up one run over four-plus innings before leaving his season debut early. With Duffey still on the roster, the Twins have a 14-man pitching staff, as they optioned outfielders Byron Buxton and Max Kepler, while calling up reliever Alex Meyer and activating utility man Danny Santana from the DL. So Duffey is a candidate to be optioned if Santana proves he's healthy, but the Twins remain in a waiting game.
"He pitched well enough for us to at least consider what he can do up here," Molitor said. "We just have to get him healthy and get him back on the mound. We look in good shape with it."
Molitor also gave updates on third baseman Trevor Plouffe, who has been on the DL since April 18 with a right intercostal strain, and closer Glen Perkins, who has been on the DL since April 11 with a left shoulder strain.
Plouffe has been making good progress and could be activated as soon as May 3, while Perkins could play catch in the next week or so. Plouffe is also a candidate to go on a rehab assignment this weekend.
"Trevor is doing really well," Molitor said. "He's really encouraged about being ready on time, which would be next Tuesday in Houston. We have a scheduled printed out for him with a progressive increase in baseball activities. We're confident about that, but it's a matter of whether we want to get him some at-bats this weekend."
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.