SAN DIEGO – Rookie seasons are about learning important lessons. Zebby Matthews learned one Monday that's so ingrained in the Twins, they used to put it on their scoreboard.

Walks will haunt.

So will errors, and Matthews made one of those, too, dropping a throw at first base in the third inning. Then the rookie righthander made matters worse by issuing two walks to load the bases. All three runs scored on Jackson Merrill's double to the wall in left-center, breaking a tie and sending the Twins to a 5-3 loss to the San Diego Padres to open a three-game series at Petco Park.

"That's what my old college coach [at Western Carolina] would say: You walk guys, hit guys, don't make them hit their way on base, they're going to find a way to score," Matthews said. "It seems to happen a lot more than when guys get a single or something. But it's just something to learn from and keep moving on."

Lots of lessons learned in this loss, which dropped the Twins to 2½ games back of the idle Cleveland Guardians in the AL Central race. Christian Vázquez got one, too, out of Merrill's big hit, which came on an 0-2 count.

"A one-pitch mistake, and we paid," said Vázquez, who has caught both of Matthews' major league starts. "The changeup [to Merrill] was up in the middle. Needed to go fastball there again. Matthews has a great fastball, and we'll learn from that."

Matthews also gave the Padres a free baserunner in the first inning, when he hit leadoff batter Luis Arraez on his right foot with a slider.

That was costly, too, when Jurickson Profar snuck a line drive just over shortstop Willi Castro's head, a ground out brought Arraez home, and Xander Bogaerts scored Profar on a two-out single.

And that's how a pitcher can give up only four hits over five innings, none of them home runs, and still give up five runs.

"Everybody's human," Matthews said with a shrug. "As much as I try [to] not walk guys and make them earn it, it's going to happen at some point. I just fell behind some hitters by not attacking, not getting ahead. It happens. It's baseball."

See, that's the crazy part: No rookie pitcher seemed to need a lesson in the danger of free baserunners less than Matthews, who made it to the big leagues by displaying the best command of the strike zone in the Twins system. Matthews, after all, struck out 49 batters in the minors this season before issuing his first walk.

Thursday's two walks actually tied Matthews' career high as a professional, including his 42 minor league appearances, and never before had he put three runners on base via walks or hit batters.

So of course the first time he does so, all three score. That's rookie luck.

"Zebby's such a good strike-thrower, but the command of his pitches was probably just a little bit off today," Twins manager Rocco Baldelli said. "The strike-throwing ability is very high-end. But what do you do when you don't have that? You learn some of those things. You learn what pitches you're still going to be able to rely on when you don't have all of them working when you want."

It might not have mattered had the Twins been able to capitalize on Michael King's own mistakes — the Padres righthander walked three and hit a batter — but only one of them scored. Trevor Larnach drew a one-out walk in the first inning and scored when Matt Wallner smacked a line drive just over first base for a double.

The Twins' second run came an inning later, and was largely due to Austin Martin's heady baserunning. After back-to-back one-out singles, Martin drew a walk to load the bases.

Willi Castro then hit what appeared to be an inning-ending double play ball to Bogaerts at second base. Bogaerts clearly planned to tag Martin out and flip the ball to first base, but Martin suddenly stopped and backpedaled, foiling that play. Bogaerts gave chase and finally tagged Martin, but too late to get Castro at first base, enabling the run to score.

BOXSCORE: San Diego 5, Twins 3

But that turned out to be the last wisp of meaningful offense the Twins would generate. They managed only one hit from the third through eighth innings, a harmless fourth-inning single that was immediately wiped out by a double play.

They were down to their final out against Padres closer Robert Suarez when Edouard Julien broke that streak and drew a walk. Julien went to second without a throw, and Vázquez drove him home with a single to left field. But Suarez got pinch hitter Jose Miranda to ground out to end the game, only the Twins' ninth loss in 27 games all-time against San Diego.