Thatcher, on the disabled list since March 22, said his shoulder passed several tests for him to get to the point where he was comfortable getting on the mound -- including successful side throwing sessions, daily treatment and two cortisone shots.
But about midway through Thatcher's bullpen session at PETCO Park, his shoulder started to bother him.
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"I think maybe it was throwing off the mound, throwing downhill ... and getting a little extension. It didn't feel good," Thatcher said. "After about a dozen pitches, it started to get weak. Then, last night, it started to hurt again."
San Diego manager Bud Black said that it's "way too soon" to know what the next step will be, though Thatcher won't pick up a ball anytime soon.
"We'll be patient with Joe, it's still early in the season," Black said. "We'll see how this plays out."
Thatcher went through similar shoulder trouble last Spring Training and started the season on the disabled list. But the shoulder responded well to treatment and he was able to rejoin the Padres in mid-April.
That won't be the case this time, though Thatcher said he's encouraged that the training staff feels they can get the inflammation out with treatment. Thatcher had an MRI last month which revealed no structural damage to the shoulder.
For now, there's just disappointment.
"It's definitely disappointing, because I felt like I was ready to go," Thatcher said. "This has been such a roller-coaster ride."