PHOENIX -- In a surprise announcement during the final week of Spring Training this year, the Padres named Eric Lauer as their Opening Day starter. That wasn't exactly a nod to Lauer as a frontline ace. San Diego had pushed three pitchers to the back of its rotation because of workload limitations, and its starting five ranked as the youngest in baseball.

Lauer, in essence, won the Padres' 2019 Opening Day job by default. A different sort of battle awaits the 24-year-old left-hander next spring.

For two years, Lauer’s rotation place has been secure in San Diego. As of Friday night, that is no longer the case. A fierce rotation battle looms in 2020.

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Lauer capped his second big league season with 4 2/3 erratic innings in the Padres' 6-3 loss to Arizona at Chase Field. Manny Machado hit his 31st homer, and Luis Urías doubled home the go-ahead run in the seventh for the Padres.

But the D-backs rallied for four runs in the eighth, tying the game on a close play at the plate in which Machado appeared to nab Jarrod Dyson on a throw home from third base. The play was overturned on a review, and one batter later Jake Lamb launched a three-run shot off Luis Perdomo, sealing the Padres’ fourth straight 90-loss season.

“It was a huge play,” interim manager Rod Barajas said of the replay ruling. “It went in their direction, changes the complexion of the next at-bat.”

Before any of that, Lauer struck out eight and allowed only three hits. But he also walked four and loaded the bases in the fifth. That’s when Barajas emerged from the dugout for a meeting on the mound.

Lauer asked for one more hitter, and Barajas granted that request. It was, perhaps, a glimpse into the philosophy of the new interim skipper. Barajas is known for putting his faith in his players, and he did exactly that on Friday night. The move backfired, however, when Lauer walked Christian Walker on four pitches.

“That’s one of those situations where you definitely want to show your manager you can get through that,” Lauer said. “I felt confident; I felt strong. I feel bad that I let him down, that I let everybody else down.”

Lauer was charged with that run and another unearned run after his exit. In 149 2/3 innings this year, he finished with a 4.45 ERA and a 4.24 FIP.

By next season, the Padres are looking to build a playoff-caliber rotation, and those numbers put Lauer squarely on the bubble.

“It's going to be a good one,” Lauer said of the looming competition. “I'm excited for it already. It's going to be the best competition we've had, and the best guys are going to come out, so you better be on top of your game."

Lauer still needs to figure out Coors Field, where he went 0-3 with a 19.13 ERA this season. But those struggles aside, he made some progress during the second half. Most notably, Lauer’s fastball velocity ticked up in September, averaging higher than 93 mph over his final three starts. He credited a mechanical fix. His whiff rate trended upward, too, as a result.

“I feel great,” Lauer said. “I think my stuff's better than it ever has been. I have the ability to throw the ball harder than I ever have. It's just really about refining things [in the offseason].”

Still, it seems likely general manager A.J. Preller adds at least one starter -- and maybe two. Garrett Richards and Dinelson Lamet have recently returned from Tommy John surgery, and top prospect MacKenzie Gore will get his crack at a roster spot in camp. Also, the Padres aren’t going to employ a six-man rotation for extended stretches, as they did this year.

In short, rotation places are going to be scarce next spring. Lauer has some work to do to earn his.