ANAHEIM -- The Angels saw some promise from top prospect Reid Detmers' first start of the season, but they were ultimately done in by their bullpen struggling mightily after his departure.

Detmers, ranked as the No. 21 prospect in baseball by MLB Pipeline, went four innings while allowing two runs on three hits and two walks with three strikeouts in a 13-6 loss to the Astros at Angel Stadium. He gave up a homer to Jose Altuve to open the game but settled down until serving up another homer in the fourth, this time to Aledmys Díaz.

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"Detmers wasn't really sharp, but he fought through it and kept us in the game," Angels manager Joe Maddon said. "We had half of the bullpen unavailable tonight, and so we tried to piece together that way, but it just did not want to work. And so they got us again. It's the second game and it's not the way we wanted to start, but we'll come back ready to play."

Detmers showed off his plus-curveball and slider, but he had trouble with his fastball command and fell behind too many hitters. The homer to Altuve came on a 3-1 fastball, while the one to Díaz was on an 0-1 fastball.

“I felt good, but I wasn’t attacking very well,” Detmers said. “I was leaving pitches up. My fastball wasn’t very good, and obviously when that happens it’s hard to pitch. I was behind in counts the whole ballgame. I got into a little groove with my offspeed, but my fastball really wasn’t there all night.”

Detmers was also hurt by a 29-pitch first inning that curtailed his ability to go deeper into the game. He was on a pitch count and was removed after four frames and 75 pitches. He left with the game tied, as the Angels scored twice in the first inning on an RBI single from Jared Walsh and a passed ball that got by catcher Martín Maldonado.

"I think he was a little amped up early on but he settled in better," Maddon said of Detmers. "But he got to 75 pitches, and for us it's hard to maintain that many bullpen innings when your starters aren't fully stretched out. But we'll get there."

But once Detmers left, the bullpen had a rough night, much like on Opening Day, when new acquisition Ryan Tepera gave up two homers in a 3-1 loss. Maddon said before the game he wants to stay away from using relievers on back-to-back days early in the season, meaning Tepera, Archie Bradley, Aaron Loup and Austin Warren were unavailable on Friday.

Maddon went to Oliver Ortega in the fifth, and he promptly gave up a run with Jose Siri dashing from third to home, even after being looked at by third baseman Anthony Rendon before he threw to first. But Walsh's throw home was way off target, allowing the Astros to take the lead. Ortega came back out for the sixth and gave up a solo homer to Alex Bregman.

"I thought [Rendon] did everything right,” Maddon said. “I was surprised Siri broke. With an accurate throw, I think he would've been out."

The game got even uglier in an eight-run seventh inning from the Astros. Mike Mayers gave up a homer to Astros rookie Jeremy Peña on his first pitch of the season and gave up three runs without recording an out. Jimmy Herget was charged with four runs while getting just one out. Long reliever Jaime Barría came in and served up a three-run homer to Kyle Tucker that gave Houston a 10-run lead, then he gave up a second homer to Tucker in the ninth.

"When we went to Mayers, that's when things fell apart,” Maddon said. "But our bullpen guys are mostly veterans, with Ortega being the only baby there, and I thought he actually did pretty well. I think Mayers and Herget will just dump it right there. And Barría did what he had to do to get us through a bad moment. Last night, our bullpen was really good but Tepera gave up a couple homers, and tonight they weren't as good."