The teams will meet for the first time since that series when they begin a three-game set Friday night in Anaheim.
Baltimore (45-83) was 32-73 before Showalter took over Aug. 3, beginning his tenure with a 6-3 home victory over Los Angeles (63-65). Center fielder Felix Pie went 6 for 11 and catcher Matt Wieters was 4 for 10 as the Orioles went on to sweep the three-game series.
The Orioles won eight of Showalter's first nine games, but have gone 5-9 since. They lost two of three on the road to the Chicago White Sox, including an 8-0 loss in Thursday's series finale.
Los Angeles, meanwhile, enters the weekend a loser of seven of its last 10 and could be headed for its first losing season in seven years.
"There's obviously some frustration right now with the way our season's gone compared to previous years, but we definitely see a better team evolving," manager Mike Scioscia told MLB.com. "So that's what we're going to work toward going forward."
The Angels enter this series on a positive note, having used a 17-hit attack for a 12-3 win over Tampa Bay on Wednesday.
"It was a good offensive day and we need more of it," Scioscia said. "They need to come out and play with focus and intensity. It's a process. It's not going to be one game, one at-bat."
Hideki Matsui capped a big series with two hits and three runs scored. He went 6 for 11 with six RBIs in the three games against the Rays, and is 17 for 33 with 12 RBIs in his last nine.
Matsui, though, went 1 for 8 earlier this month against the Orioles, and is hitting .167 over his last 16 games against them.
The slugger is 0 for 3 in his career against probable Baltimore starter Brad Bergesen (5-9, 5.84 ERA), who has never faced the Angels.
The Orioles have won Bergesen's last four starts, with the right-hander going 2-0 with a 3.18 ERA in that span. He yielded five runs over seven innings and did not get a decision in Saturday's 8-6 home win over Texas.
"I feel great," Bergesen said "I felt like I just battled all day - didn't necessarily have my best stuff, but tried to go as deep as I could."
Trevor Bell (1-3, 5.48) will make his fifth start for the Angels after going 0-2 with a 6.38 ERA in his first four. He gave up three runs over 4 1-3 innings Saturday in a 9-3 victory at Minnesota.
The right-hander is making his first home start. Bell faced Baltimore on Aug. 3, allowing two runs over 5 1-3 innings and not receiving a decision.