SAN DIEGO -- Joe Thatcher, who was one of the top left-handed specialists in the National League last season, had a setback with his sore left shoulder in a bullpen session on Tuesday and will be sidelined indefinitely.

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."

Maybin still testing vast center field at PETCO

SAN DIEGO -- It was an unfair question, one new Padres center fielder Cameron Maybin hesitated before answering.

What does Maybin think of the playing the vast center field of PETCO Park, a ballpark that rates as pitcher-friendly with a lot of room to run?

"That's tough to say ... I don't know, I've been playing center field so long that all that I care about is when they hit it, I go get it," Maybin said. "I don't really think about it as 'I have a lot of space to cover.'

"Maybe in two or three weeks or two or three months, I'll be able to answer the question better."

Maybin was tested several times in Tuesday's home opener against the Giants, tracking down a Buster Posey fly ball deep in center field in the first inning. He also caught line drives by Freddy Sanchez in the seventh inning and Pablo Sandoval in the eighth.

"There's a lot of room to roam out there," Maybin said.

One of the reasons the Padres obtained Maybin was because they felt he was a good fit to play center field at PETCO Park, part of their offseason initiative to strengthen the team up the middle.

Patterson tests strained left hamstring

SAN DIEGO -- Under the watchful eyes of Padres manager Bud Black, utilityman Eric Patterson tested his strained left hamstring by undergoing a series of on-field tests two hours before Wednesday's game against the Giants.

Patterson, on the disabled list since March 22, ran sprints in the outfield, ran the bases, took batting practice and received fly balls in the outfield and several ground balls at second base.

The consensus was that Patterson is ready to test his hamstring in games, which is why he will head one hour north of San Diego to play with Class A Lake Elsinore for several days on a Minor League rehabilitation assignment -- unless his hamstring tightens up late Wednesday or early Thursday.

"He felt fine," Black said.

Patterson was eligible to come off the disabled list Wednesday, and it's believed he will not need more than a handful of Minor League games before he's activated.