Middleton entered in the ninth inning Friday, allowing one run on two hits and recording his first career strikeout.
Middleton was in Reno with the Angels' Triple-A affiliate on Thursday when he received a late-night phone call informing him of his promotion. He flew to Los Angeles on Friday morning, marking the beginning of a whirlwind day for the 23-year-old right-hander.
"I've been here for a while, settling down," Middleton said before the game. "I didn't have anything ready. I don't have any red stuff."
The Angels drafted Middleton in the third round of the 2013 Draft out of Lane Community College as a starter, but he struggled in the rotation and finished the '15 season with a 5.30 ERA at Class-A Burlington. In '16, Middleton shifted to the bullpen, where his velocity ticked up and ignited his rapid ascent through the Angels' farm system. He climbed all the way to Triple-A Salt Lake, where his fastball was reportedly clocked at 102 mph.
Overall, Middleton logged a 3.41 ERA with a 1.14 WHIP and 88 strikeouts in 66 innings across three Minor League levels last year.
He earned an invitation to Major League camp during Spring Training, but he logged a 7.71 ERA in seven Cactus League appearances. Still, Middleton had been pitching well for Triple-A Salt Lake in recent weeks, recording a 2.84 ERA over 12 2/3 innings.
"Keynan is a kid that came into spring with high expectations and really never found himself in Spring Training," manager Mike Scioscia said before his debut. "He never really got comfortable on the mound. He worked hard on a lot of things, but he made some adjustments the last couple of weeks down there, and you saw the stuff re-emerge. This guy's got a big arm. We feel he's ready for the challenge. There's always a little bit of seasoning that goes into a young pitcher getting where he needs to be, but we're ready to start the process."
The Angels hope that the addition of Middleton will help boost their bullpen, which has been hit hard by injuries to Cam Bedrosian, Huston Street, Andrew Bailey and Mike Morin. To clear a spot for Middleton on the roster, the Angels optioned right-hander Daniel Wright, who allowed five runs over four innings in the Halos' 11-3 loss to the Mariners.
• Left-hander Tyler Skaggs said he was disappointed to learn of the severity of his oblique strain, which is expected to keep him sidelined for 10-12 weeks.
"It's the most frustrated I've ever been in my career because I felt like I was throwing the ball well," Skaggs said. "I feel like I'm back to the pitcher that I used to be in 2014. I put in a lot of hard work in this offseason to come in ready to go, and it's kind of a freak thing that happened, so I'm not too happy about it."
• Center fielder Mike Trout returned to the Angels' lineup on Friday after missing one game with hamstring tightness. Right fielder Kole Calhoun, meanwhile, received a day off on Friday to "recharge," Scioscia said.