Red Sox closer Kenley Jansen will not pitch during Boston’s series against the Rockies in Colorado. Jansen has atrial fibrillation and the higher elevation has caused problems in the past. Jeff Dean/Associated Press

The Boston Red Sox suddenly have a bullpen issue.

The team arrived Sunday night in Denver for a series against the Rockies without closer Kenley Jansen, whose 439 career saves is fifth all-time. Jansen is skipping the series because of an atrial fibrillation heart condition, which became a concern after previous trips to the Mile High City.

That’s why he was on the mound in the eighth inning Sunday night, surprisingly appearing in a game the Sox trailed by three runs. He had pitched just a day earlier, blowing a save for the first time in nearly three months when his former Red Sox and Dodgers teammate, Kiké Hernández, hit a tying home run in the ninth inning that led to an 11-inning victory for Los Angeles.

Jansen was back in for Sunday’s game, hoping to give his fellow relievers a little rest before they arrived in Colorado a pitcher short. He won’t join the team until Friday’s home game against the Yankees.

“We knew this in spring training,” Manager Alex Cora said. “We talked about it before the series. Before the game he was like, ‘I’m pitching regardless if we’re up or we’re down. My job is to help the bullpen and save the bullpen.’”

It didn’t work as planned. Jansen gave up three more runs in a 9-6 loss that gave Boston its first three-game losing streak since April.

The sweep makes you wonder if Criag Breslow, the chief baseball officer, will add any bullpen arms before the July 30 trade deadline. It didn’t seem high on the team’s list of needs, but with Chris Martin and Justin Slaten sidelined, the bullpen is a little thin. And that was before the team had to spend three days without Jansen.

This is the time of year when small leaks can become major problems. The Red Sox rolled into the All-Star break bursting at the seams with good vibes. Boston went 10-3 going into the break, providing a reason for optimism.

The Red Sox also need to add a starting pitcher. Under pitching coach Andrew Bailey, the rotation has been one of the best in the American League, but remains thin on big-game experience; the staff has one combined playoff start.

It also doesn’t take away from Boston’s need for at least one right-handed hitter. After Sunday night’s loss, the Sox are 12-17 against left-handed starters. They have struck out more against lefties than any other team.

The Sox aren’t the only contender with flaws, of course, but their blemishes were on full display over the weekend in Los Angeles. And that couldn’t have come at a worse time for a team trying to convince its front office to go all in on a 2024 playoff push.

Tom Caron is a studio host for the Red Sox broadcast on NESN.