Featherston was diagnosed with a strain in his upper back and Rasmus has a strain in his right forearm, injuries that shouldn't keep them on the shelf much longer than 15 days. The Angels needed two fresh bullpen arms because Shoemaker gave up six runs in Saturday's second inning, prompting relievers to account for 6 1/3 innings of a 9-4 loss to the Royals.
One year after finishing second in American League Rookie of the Year Award voting, Shoemaker is 5-9 with a 4.76 ERA. The 28-year-old right-hander briefly turned his season around, posting a 1.69 ERA from June 26 to Aug. 4, but he's given up 13 runs in 7 1/3 innings over his last two starts.
The Angels will need a fifth starter on Thursday, but Nick Tropeano -- currently slated to start Wednesday's game for the Salt Lake Bees -- can easily be pushed back to make a spot start that day.
"I'll make the best of it," Shoemaker said of going down to the Minor Leagues. "I kind of put myself in this position. If I pitched better, it's not an issue. That's the way I look at it. I'll go down and fix some things and get back here as quick as I can."
Aybar sustained his injury while diving for a popup in Chicago on Wednesday. The 31-year-old switch-hitter hoped to return at Angel Stadium on Monday, but he felt good enough while taking swings during batting practice and decided to come back a day early.
"I want to play," Aybar said. "This team is struggling a little bit."
Rasmus has been dealing with forearm issues "on and off," issues that perhaps showed up while giving up four runs in four innings over the last six days.
"I don't think it'll be a problem," Rasmus said of his ailment, "but we needed some guys now."
Featherston has been nursing pain in his upper back since a rib popped out while pinch-hitting against Orioles reliever Darren O'Day in the eighth inning of last Sunday's game. He took a couple of pain-relieving injections while the team was in Chicago earlier this week and played through some discomfort while filling in for Aybar over the weekend.
"It's been a physical grind," Featherston said. "I've never had an issue like this before."
Featherston's stint on the DL gave the Angels an opportunity to call up a shortstop Angels manager Mike Scioscia believes is "a little more polished." Jackson, 27, spent 20 games in the Majors with the Cardinals from 2012-13 and was acquired from the Royals for backup catcher Drew Butera on May 7.
Jackson, who can also play second and third base, posted a .295/.372/.378 slash line for the Angels' and Royals' Triple-A affiliates.
"Defensively, I'm solid; I can handle the bat," Jackson said when asked to assess his strengths. "I'm a guy who plays to win."
The Angels, who have lost 16 of their last 22 games heading into Sunday, have a complicated roster situation with September still a couple of weeks away. Their 40-man roster is full -- C.J. Wilson was transferred to the 60-day DL to create room for Jackson -- and it'd be hard for them to option any of their position players to bring up an extra reliever.
Their bullpen has had to absorb 22 2/3 innings in the first six games of this road trip.
Shoemaker was a casualty.
"It's never an easy decision or conversation, but we all feel very strongly that it's the right move to make right now for Matt and for us," Scioscia said. "Hopefully, he'll emerge on the other side of this as a pitcher closer to what we need."