Ryan Brasier, center, is pulled by Manager Alex Cora, right, during the seventh inning after allowing three unearned runs in Boston’s 5-4 loss on Monday in Seattle. Ted S. Warren/Associated Press

SEATTLE — Since the very beginning of spring training, Red Sox Manager Alex Cora has preached the importance of improved team defense, even going so far as to revamp the club’s facility in Fort Myers, Florida, to put a greater emphasis on glove work during camp. Yet as the regular season winds down, the Red Sox remain a below-average defensive team – and it’s proving costly as they try to chase down a postseason spot.

On Monday night, two key errors led to four Mariners runs in a game Seattle won, 5-4. In the second inning, Luis Torrens advanced to second on a single after Hunter Renfroe misplayed the ball on one hop in right field and would come around to score on a Tom Murphy RBI groundout. With two outs in the seventh inning of a 2-2 game, first baseman Kyle Schwarber booted a Jake Bauers grounder to extend the inning; Mitch Haniger hit a three-run homer to give Seattle a 5-2 lead two batters later.

The four unearned runs the Red Sox allowed Monday put them at 10 in the last three games. They have given up 33 unearned runs since the All-Star break – the most of any American League team.

“Like I always say, you give the opposition more than 27 outs, most of the time they’re going to take advantage,” said Cora. “It seems like right now, whenever we open up the window or the door, whatever you want to call it, they take advantage and we pay the price.”

Schwarber’s misplay could not have come at a worse time for Boston. After Eduardo Rodriguez went six strong innings, Ryan Brasier took over in the seventh and needed just five pitches to retire the first two batters. Bauers, pinch-hitting for Dylan Moore, then hit a chopper right at Schwarber, but the ball deflected off his glove. J.P. Crawford then singled to put two men on for Haniger, who deposited a Brasier slider over the left-field wall to put Seattle up by three.

“It was in the glove, squirted out, and that was it. I’m frustrated,” Schwarber said. “I wanted to make that play. It’s a 1-2-3 inning and it turned into a lot more.”

The Red Sox acquired Schwarber with the intention of having him play a good amount of games at first base, but Bobby Dalbec’s sudden improvement on both sides of the ball has led to more playing time for the rookie at that position. Monday was just Schwarber’s fourth start at first base – and first in six days. For at least a moment, the veteran looked like someone who was learning a new position on the fly.

“I knew the day was going to come where I was going to make an error,” he said. “It’s inevitable to happen. It’s unfortunate that it turned out to be the thing that kind of turns the tide and makes it sting a whole lot more… there’s no excuses.

“The next time that happens, I want the ball to be hit to me again,” he added. “I’m not shying away from it. I’ll let you know I messed up. It should have been a play that was made and it wasn’t.”

With 16 games to go, Cora can only hope his team can make enough plays to keep itself in games. With five teams within 3 1/2 games of each other in the wild-card race, one play could be the difference between a postseason berth or an early start to the winter for the Red Sox.

“We’ve been struggling defensively the whole season,” Cora said. “We had some good stretches when we play good defense. When we do that, most of the time it seems like we win games. That’s the bottom line. At this level, you have to be good defensively.”