ANAHEIM -- While the Angels have been beating up on the contending Mariners over the past two weeks, winning six of seven against them, the last-place A’s have been giving the Angels fits recently and it continued in Thursday’s series opener.

The Angels jumped out to an early lead with a five-run second inning, but lefty Kenny Rosenberg couldn’t hold it, as he struggled in his first start of the season in a 6-5 loss at Angel Stadium. It marked their sixth defeat over their past seven matchups with Oakland, dating back to July 2. And Rosenberg, who gave up six runs 3 1/3 innings, could be a candidate to be optioned if the Angels think they need relief help after his short outing.

“I mean, I lost that game for us, right?” Rosenberg said. “Elephant in the room. I gave up six, we lost 6-5. Ultimately, it feels bad to lose a game for a team playing really well recently. So for me to give up six runs in three-plus innings, it’s not good enough and not good enough for the team.”

With the rotation thin, Rosenberg was called on to start after he pitched well in long relief against the A’s on Saturday, limiting them to one run over four innings. He replaced prospect Jack Kochanowicz after he was optioned because he posted a 14.14 ERA in his first two career starts.

Rosenberg, though, failed to replicate his success from Saturday and scuffled from the start to put the Angels in an early hole. Lawrence Butler and Brent Rooker have combined to torture the Angels this season and they kept it up in the opening frame. Butler doubled to open the game, stole third and scored on a throwing error from catcher Logan O'Hoppe. Rooker then deposited a 2-1 changeup over the heart of the plate over the bullpens in left field for a solo shot.

“When we make mistakes in the wrong part of the plate with pitches, they’re not missing them,” Angels manager Ron Washington said. “Rosenberg threw a changeup to Rooker and he didn’t miss it.”

The Angels, though, rallied in the second against veteran right-hander Ross Stripling. Kevin Pillar sparked the big inning with a one-out double before O’Hoppe plated him with an RBI single and went to second on the throw. Zach Neto tied it with an RBI single of his own before Mickey Moniak put them ahead on a sacrifice fly. Luis Rengifo provided a key two-run double to left that Miguel Andujar misplayed with two outs. Andujar later made up for it by throwing out Moniak trying to stretch a double into a triple in the seventh.

But most importantly, Rosenberg couldn’t protect the three-run cushion. Butler crushed a two-run homer in the third on a 2-0 cutter at the bottom of the zone. In the fourth, he gave up a hard-hit double to Tyler Nevin before surrendering a two-run blast to Daz Cameron to give the A’s the lead. Rosenberg again paid for falling behind in the count, as Cameron's blast came on a 3-1 fastball right down the middle.

“In a nutshell, Major League hitters hit mistakes, right?” Rosenberg said. “You throw a pitch over the heart of the plate, whether you're ahead or behind, they're ready for that kind of stuff. But even more nuanced than that, it's counts and locations. If you're ahead in the count, you get away with a little bit more but if you’re behind in the count, you get away a little bit less.”

That was it for Rosenberg and the Angels will have to determine how they’ll handle his spot in the rotation next time up and they’re not exactly brimming with options.

Lefty Reid Detmers started for Triple-A Salt Lake on Wednesday, allowing three runs (one earned) over five innings and has a 6.49 ERA in eight starts in the Minors. Right-hander Chase Silseth allowed one run over 4 1/3 innings with Triple-A Salt Lake on Wednesday but walked three and is still building back up after experiencing a slight setback with his elbow. And right-hander Sam Bachman started with Double-A Rocket City on Sunday, giving up five runs and walking seven over 2 2/3 innings. But they did sign veteran Johnny Cueto to a Minor League deal on Wednesday to provide depth after he opted out of his deal with the Rangers earlier this month.

“We haven’t discussed any of that yet,” Washington said. “Right now, we’re thinking about tomorrow.”