KANSAS CITY, Mo. — How do you figure this one?

The Red Sox had issued the fewest walks in baseball (181) and the Royals had drawn the fewest (169). The Sox bullpen had surrendered only 19 homers, second fewest in the majors. So how do the Royals rally in the eighth inning Wednesday afternoon for a 6-4 victory over the Red Sox on a grand slam set up by three walks?

Chalk it up to “one of those days.’’

Righthander Matt Barnes relieved Joe Kelly to start the eighth inning with the Red Sox clinging to a 4-2 lead. He quickly walked Jorge Bonifacio and Lorenzo Cain, throwing just two strikes in his 10 pitches. In came lefthander Robby Scott to face the lefthanded-hitting Eric Hosmer, but Scott missed with four pitches to load the bases. Manager John Farrell remained in the dugout as righthanded-hitting catcher Salvador Perez came to the plate. Perez worked the count to 3 and 2 and fouled off three pitches before driving the ninth pitch of the at-bat 412 feet to left field for his 15th homer of the season.

Where was closer Craig Kimbrel? Farrell has used Kimbrel five times this season for more than one inning, had decided Wednesday wouldn’t be his sixth appearance.

Farrell also noted Perez’s numbers this season against lefthanders (.242 batting average, .394 slugging percentage, .676 OBP). The Sox bullpen also had issued the walks in baseball heading into the game (70 in 215⅓ innings).

So this was a head-scratcher.

OK, bullpen failures happen. In fact, it was the second loss of the three-game series in Kansas City for the bullpen. And the pen did surrender eight runs in 8⅓ innings after giving up just one run in their previous eight games.

Maybe this is just a bullpen slump — every team has one. Ask the Yankees, who entered Wednesday on a seven-game slide in which their bullpen had given up 16 earned runs in 26 innings (5.54 ERA). All in all the Sox bullpen has been the strength of the team. Kimbrel is the best closer in baseball right now, and Kelly has been rock solid in a setup role.

Sometimes numbers don’t tell the whole the story — even when you’re the No. 2-ranked bullpen and you had posted a 1.11 ERA in your last 12 games and a 2.80 ERA overall, history-making numbers for the franchise.

But Wednesday’s final results stunk. It nullified a strong effort by starter Drew Pomeranz, who pitched two stellar games on this trip but came away with a pair of no-decisions. Pomeranz has held opponents to two or fewer runs 11 times in 14 starts — keeping 2017 company with Clayton Kershaw, Max Scherzer, Ariel Miranda, and Gio Gonzalez. Nice company.

But Pomeranz was done in by a blown save and Farrell’s decision to hold back Kimbrel.

“Given the workload [Kimbrel] had and how he was feeling coming out of those, that wasn’t something I was looking to entertain,’’ Farrell said. “[Perez’] power numbers have come against righthanded pitching . . . It didn’t happen today.’’

Farrell also has been very careful with Kelly’s arm, not pitching him in back-to-back days given the hard-throwing righthander’s past arm and elbow issues.

Barnes has been solid in the eighth-inning role, but he just couldn’t find the plate.

“We uncharacteristically lost the strike zone,’’ Farrell said. “For a group that has been so good in not issuing walks throughout the year, we had an inning that got away from us. Matt couldn’t keep the ball down. This one stings because that group has been so good, so consistent for the better part of the season.

“It’s certainly not a trend. It’s not something we’ve dealt with very often.’’

It happens to everyone.

No team likes to lose games late. The Red Sox were 29-0 when leading after seven innings before this meltdown.

Still, the silver lining was Pomeranz

If he continues to pitch well, the Red Sox will have a valuable back-end starter capable of shutting down big lefthanded lineups and perhaps staking claim as the best No. 4 or No. 5 starter in baseball.

Pomeranz went into the seventh inning for the second straight start and appears to be overcoming his early-season problem of high pitch counts before the fifth inning — a killer for the Red Sox bullpen.

So as the Red Sox plug one hole, two other holes open. Hanley Ramirez’ shoulders are preventing him from swinging the bat properly, so he took Wednesday off. And now the bullpen let them down.

Hopefully this is an isolated incident and they don’t fall into the bullpen crisis the Yankees now face.

Nick Cafardo can be reached at cafardo@globe.com. Follow him on Twitter @nickcafardo.