"We're going to slow him down just a touch," Padres manager Bud Black said. "Better safe than sorry."
Thatcher said on Friday that he would likely back off throwing for the foreseeable future.
"I felt great all spring. ... I threw BP yesterday and it really stiffened up," said Thatcher, who indicated the training staff was going to treat his shoulder aggressively. "It hurt pretty good this morning."
2010 Spring Training - null
Sights & Sounds
Spring Training Info
Thatcher said it's the same area that affected him a year ago during Spring Training -- an injury that was termed a shoulder strain. He was limited to only two innings of work last spring because of the injury.
Thatcher subsequently began the regular season on the disabled list before joining the Padres on April 22 upon completing a rehabilitation stint with Triple-A Portland.
Thatcher, who had a 1.29 ERA in 65 games last year, was scheduled to throw one inning in the Padres' Cactus League opener on Monday against the Seattle Mariners.
The Padres signed veteran left-hander Randy Flores to a Minor League deal earlier this month and invited him to camp.