SAN FRANCISCO -- It was a short visit, in front of an empty ballpark, but the 25 pitches that Joe Thatcher threw Monday were the most meaningful ones he's thrown in a very long time.

Thatcher, two months and two days removed from surgery on his left shoulder, threw off a mound for the first time since February in Spring Training.

The throwing session was an important step in his rehabilitation, mentally and physically.

"This was sort of my last mental hurdle ... getting on the mound again," Thatcher said. "It was exciting and just fun to be back out there, throwing and visualizing hitters."

Thatcher threw 25 pitches, all fastballs, to bullpen catcher Justin Hatcher. Manager Bud Black and pitching coach Darren Balsley watched from the side.

It was May 2 when Thatcher had surgery to shave down his frayed labrum and rotator cuff and to remove the bursa sac in his left shoulder.

Thatcher was sidelined most of Spring Training because of soreness in his left shoulder and then started the season on the 60-day disabled list, retroactive to March 22.

Before Monday, the last time Thatcher stood on a mound was in late February in Spring Training while throwing batting practice to Padres hitters. That's when he first started to feel pain in his shoulder.

Thatcher will throw another bullpen session Wednesday as he progresses during his rehabilitation that will eventually lead him to side sessions and then some time pitching in the Minor Leagues on a rehabilitation stint.

"That's a big step. ... In a sense, this is his Spring Training," Black said. "There are still a number of hurdles he has to clear but this [Monday] was a big one."

There's no timetable to when Thatcher -- who last season was one of the top left-handed specialists in the National League, posting a 1.29 ERA in 65 games -- will return, nor are the Padres anxious to push for a speedy return.

Gregerson set for return to action

SAN FRANCISCO -- Luke Gregerson rejoined the Padres on Monday morning, although he won't be reinstated from the disabled list until after the game or Tuesday.

Gregerson, who has missed 24 games with a right oblique strain, made his second Minor League rehabilitation assignment Sunday with Triple-A Tucson, which was playing in Sacramento.

Gregerson struck out two in one scoreless inning, though the results of his two outings with Tucson weren't something that concerned manager Bud Black.

"He felt good. The thing that's important is the physical," Black said.

Gregerson allowed three runs on three hits and two walks while getting one out Friday while pitching for Tucson.

Black said the Padres would either reinstate Gregerson from the disabled list after the game Monday against the Giants or before Tuesday's game. There's a small chance Gregerson will need another rehab appearance, Black said.

Gregerson was placed on the disabled list June 9, retroactive to June 7.

He first felt some pain in his left side while warming up in the bullpen in a game on June 8 against the Rockies and immediately stopped.

At the time of the injury, Gregerson was 2-1 with a 2.63 ERA in 30 games.

Padres prospects earn promotions

SAN FRANCISCO -- It was moving day Monday for a handful of the Padres' top Minor League prospects.

Third baseman James Darnell and closer Brad Brach of Double-A San Antonio earned promotions to Triple-A Tucson.

Also, third baseman Jedd Gyorko and pitcher Miles Mikolas moved from Class A Lake Elsinore to San Antonio.

A handful of other players the Padres regard highly have already earned promotions, like first baseman Anthony Rizzo (Tucson to San Diego) and reliever Josh Spence, the first player from the Padres' 2010 Draft to reach the Major Leagues.

"It's really been exciting to watch the progress of so many guys in the first half," said Randy Smith, the Padres' director of player development.

"Not only the guys promoted today but the guys who have made their Major League debuts and many others who have yet to be promoted."

Gyorko, the Padres' second-round pick in 2010, was hitting .365 with 18 home runs, 35 doubles and 74 RBIs. Mikolas, a seventh-round pick in 2009, was 3-0 with a 1.13 ERA and 12 saves with Lake Elsinore.

Darnell was hitting .333 with 17 home runs, 25 doubles and 62 RBIs for San Antonio. Brach, who earned his 100th professional save Friday, was 2-2 with a 2.25 ERA and 23 saves for the Missions. Brach is coming off seasons where he had 33 and 41 saves.