Jim Baumbach Newsday columnist Jim Baumbach

Jim Baumbach is an investigative / enterprise sports reporter for Newsday. A Long Island native, he started working for Newsday in 1998. Show More

If this 2-0 loss to the Dodgers is the one that keeps the Yankees out of the playoffs, it will be easy to look at Starlin Castro as the primary culprit.

The Yankees second baseman failed to catch a routine liner off the bat of the Dodgers’ Corey Seager leading off the ninth inning, and everything went downhill from there.

“That’s an easy out,” he said. “That’s one of the plays I always make.”

But over time Castro’s gaffe will look nothing more than a play he missed, inexplicably. Bad timing, yes. But errors happen. Instead, the biggest takeaway here is that Castro’s error once again exposed the curious case of Yankees closer Dellin Betances.

The 6-8 righthander is a strikeout machine, with a 97-mph fastball and 84-mph pitch that’s part slider, part curveball. Often, he’s downright unhittable.

But put a runner on first base and, well, suddenly Betances morphs into what scouts like to call a top-step closer. As in, the manager has one foot always on the top step, anxious about what’s going to happen on every pitch.

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Betances’ inability to hold runners could dog him as a closer, because once someone is on base the game suddenly changes in the offense’s favor.

Take Seager, for example. The Dodgers’ all-everything rookie shortstop, who was on first base after Castro’s error, is anything but an aggressive baserunner. Before Wednesday, Seager had attempted only six steals in 167 major-league games. He was caught stealing three times.

Still, with Betances on the mound, it doesn’t matter who the runner is on first base. The smart play is to try for second, especially in a tight game.

Clearly the righthander is uncomfortable making the routine pickoff throw to first to keep runners honest and his hulking frame makes his throws home all the more time-consuming.

What was most surprising Wednesday was that it took Seager five pitches before he tried for second. And that when he finally did go, catcher Gary Sanchez almost threw him out even though the pitch was in the dirt because Seager was so cautious with his jump.

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Yet the result was the same as always. It was the 19th time a runner tried to steal a base with Betances on the mound. They’ve succeeded every time.

On cue, Turner lined the next pitch into leftfield, easily plating Seager from second with what proved to be the only run that mattered.

Betances’ night only got worse from there.

After Turner advanced to third on a flyout to deep leftfield by Adrian Gonzalez, Yasmani Grandal hit a dribbler back to the mound. Turner took off for home on contact.

It should have been an easy play for Betances. It wasn’t.

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He didn’t set his feet, he didn’t have a grip on the ball and he rushed his throw home. It sailed over Sanchez’s glove. Turner scored easily.

“It probably would have been better,” he said, “if I threw it underhand.”

It was only a few years ago when many in the Yankees front office wondered if Betances — a former eighth-round pick they invested first-round money in — would ever be able to harness his fastball. He has since turned himself into one of the most electric relievers in the game.

But in order for Betances to take the next step and become a lockdown closer, he needs to figure out how to grow comfortable in settings that have nothing to do with balls, strikes and missed bats. He needs to learn how to hold runners and make routine throws.

Maybe it’s nothing more than a mindset.

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“I think it’s more just trusting that it’s going to be good,” Betances said, “instead of thinking it might not be.”

The Yankees have little room for error. By trading away Aroldis Chapman and Andrew Miller, they made a decision to trust Betances in these big situations.

He has to trust himself, too.