Primarily a second baseman, Amarista (No. 12) and Roach (No. 16), a right-handed pitcher, were rated among the Angels' Top 20 prospects by MLB.com.
"[General manager] Josh [Byrnes] felt players with the players coming back our way, you have two young guys who can have an impact on us in the future," Padres manager Bud Black said. "And with the middle infielder [Amarista], possibly relatively soon."
Amarista, 23, was hitting .273 with eight extra-base hits and 12 RBIs in 18 games this season for Triple-A Salt Lake, the Angels' top Minor League affiliate. He made the club's Opening Day roster and appeared in one game as a pinch-runner before being optioned to Salt Lake. He also appeared in 23 games for the Halos last season, hitting .154.
Amarista, who has also played shortstop and the outfield, originally signed with the Angels as a Minor League free agent in January 2007. He's a .314 hitter over parts of six Minor League seasons. He will join Triple-A Tucson on Friday.
"He's a smaller guy, but he's a guy who has hit above .300, is hard to strike out, can run a little bit and is a good defender," Byrnes said. "We like his ability to play a number of positions."
With the contracts of shortstop Jason Bartlett and Orlando Hudson ending after this season, the Padres are looking for depth up the middle. Bartlett does have a $5.5 million option for 2103 that vests if he reaches 432 plate appearances.
"For the short term and the long term, it's an area we want to improve," Byrnes said.
Byrnes cited the recent improved play of shortstop Everth Cabrera with Tucson and that infielder Logan Forsythe is starting to play in games again after breaking a bone in his left foot in Spring Training.
Roach, 22, was 5-0 with a 2.16 ERA and 29 strikeouts in 41 2/3 innings with Class A Inland Empire. He is currently leading the California League in victories. Roach was a third-round Draft pick in 2010 who has amassed a 14-6 record with a 3.97 ERA in his first 66 professional games in three seasons.
Roach will remain in the California League, likely joining Lake Elsinore on Friday. His fastball runs in the mid-90s.
"Roach has a very good sinker and was one of the best ground-ball guys in the Minors," Byrnes said. "It's a good arm to add to the system."
Frieri, the longest tenured player in the organization, was 1-0 with a 2.31 ERA in 11 appearances in 2012. He signed with the team as a 17-year-old in January 2003 and reached San Diego in '09. He was 3-3 with a 2.33 ERA in 105 relief appearances for the Padres.
"Frieri was throwing very well and our bullpen has been strong, but as banged up as we have been, we have some inventory with right-handed relievers," Byrnes said. "We were not looking to change our bullpen, but we have some depth there."
The Padres will add a player to their 25-man roster on Friday, presumably a relief pitcher. Brad Brach is one candidate, having already pitched for San Diego this season.
The Padres, after consecutive victories over the Brewers, are 9-17. The team was off on Thursday before the start of a three-game series against the Marlins on Friday at Petco Park.