Hector Santiago thought he was 100 percent again after coming off the disabled list last week, and said his shoulder seemed fine even after two disappointing starts. But that changed Tuesday, Paul Molitor said.

"He went out and threw a [bullpen session], and when he came back in, he went in to see our training staff. He said something wasn't right," the Twins manager said. "He's dealing with some things in his upper mid-back and his posterior shoulder, and that causes a little more concern than last time, when we shut him down for awhile."

The Twins send the lefthander for a magnetic resonance imaging test Wednesday night to see whether there is a more serious physical problem. The tightness isn't necessarily related to the injury that landed him on the disabled list after his June 6 start in Seattle. But Santiago also hasn't been the same pitcher since mid-May — he had a 2.76 ERA on May 9, and a 9.96 ERA and seven consecutive losses since then — and the Twins are wondering whether there's a physical reason for the change.

"After watching him throw [Tuesday, bullpen coach Eddie Guardado and pitching coach Neil Allen] said it wasn't great but it wasn't bad. But he felt something, and we have to address that," Molitor said. "I've talked to people who try to fight their way though those things. Players can have great intentions, but in reality, it sometimes can be a detriment to your team when you can't do what you're capable. … [Guys] have to understand they're putting their careers at risk a little bit more on the pitching side."

With Santiago sidelined again, righthanded reliever Alan Busenitz, who has a 2.08 in six appearances with the Twins, was recalled from Class AAA Rochester. But the Twins will need a substitute starter for Friday's game against Baltimore. Felix Jorge, who earned a victory in his five-inning MLB debut Saturday, seems to be the logical candidate.

Mauer sits out

The Fourth of July included a few fireworks in Joe Mauer's lower back. Or at least, that's what it felt like to the veteran.

Santiago

Mauer, who walked in the first inning of Tuesday's victory over the Angels, wrenched his back while running the bases, producing pain in his back that kept him out of action a day later.

"I felt it yesterday" while running the bases, Mauer said after sitting out Wednesday's 2-1 loss. "I was able to get through it. But I'm paying for it now."

Mauer wasn't in the Twins' lineup Wednesday — coincidentally, as Brian Dozier returned to action after missing two games because of back pain of his own — and was too sore to take batting practice.

But it became clear that Mauer was unavailable when manager Paul Molitor couldn't call on him to pinch-hit in the ninth inning of a 2-1 game, with the bottom of the order coming up.

"I'll just say he's not 100 percent today," Molitor said. "He bounces back quickly, though. We'll see how he responds tomorrow."

Etc.

• Lefthanded reliever Craig Breslow will throw in the bullpen Thursday, Molitor said, then pitch an inning for Rochester on Sunday as the team decided how soon to activate him from the disabled list. Breslow went on the DL last Wednesday because of soreness in his rib cage.

• Former closer Glen Perkins will throw off a mound Thursday as he begins building up to rehab games again after a setback with biceps tendonitis in his left arm.