Sanchez said he was feeling better Tuesday, but he experienced pain while throwing Monday. Manager Brad Ausmus said Sanchez will not be able to return for Saturday's doubleheader against the Chicago White Sox.
"With the situation we are (in) right now, of course I'm concerned," Sanchez said. "I don't like being on the bench, just seeing everything happen. I know the team is going to do very well, we're going to start playing better. Personally, I'm a very competitive person, and I like being on the mound all of the time."
Detroit has struggled this month amid injuries to Sanchez and Justin Verlander, falling out of first place in the AL Central. Spot starters Robbie Ray and Buck Farmer failed to get out of the second inning in their most recent outings.
Sanchez seemed to be making progress before his latest problem Monday.
"He actually backed up to about 150 feet, was throwing good, felt good. Then came back in at 60 feet to do a little flat ground, bullpen-type thing," Ausmus said. "It wasn't until he threw a changeup that he felt it."
Lobstein made his major league debut in relief last weekend, allowing three runs in 5 2-3 innings.
Detroit looked like one of the World Series favorites when the Tigers traded for left-hander David Price at the deadline, but injuries have plagued the pitching staff in August. Detroit is also missing reliever Joakim Soria, who has a left oblique strain.
Soria was expected to throw Tuesday.
Sanchez was the American League's ERA champion last year, but he's had a hard time staying healthy in 2014. He went on the disabled list in late April with a lacerated finger, and he's made only 21 starts, six fewer than teammates Price and Max Scherzer.
"Right now, just wait until I got a full range of motion, and pain free," Sanchez said. "When this happens, I start to throw again."