Xander BogaertsBOSTON — The Red Sox’s losing streak extended to six games Friday. And it was their most crushing loss yet.

Koji Uehara allowed five runs in the ninth inning as the Red Sox fell to the Seattle Mariners 5-3 at Fenway Park. It was Boston’s first loss of the season when leading after eight innings.

The Red Sox had been 44-0 when entering the ninth inning with a lead. They were the only team in Major League Baseball that had not suffered a loss when leading after eight innings.

The Red Sox will need to put Friday’s devastating defeat behind them, as Chris Young and Hisashi Iwakuma — two pitchers having great seasons — wait in the wings this weekend.

— Red Sox starter Joe Kelly left after five shutout innings for precautionary reasons. The right-hander felt a tweak in his shoulder and the Red Sox didn’t want to run the risk of him injuring himself.

“It stinks. I had a little minor tweak in my shoulder that I felt on a curveball in the first pitch of an at-bat, and it’s something that I haven’t felt,” Kelly said. “I had a couple of more warm-up pitches where I thought I was OK enough to finish the inning and then was taken out due to precautionary reasons. It’s something that me and the training staff will look over and we think it’s not too serious.”

Kelly said he doesn’t expect to miss his next start.

— Kelly pitched pretty well over his five innings, too. He struck out five and showed good velocity.

— Xander Bogaerts nearly launched his ninth home run of the season in the second inning.

Bogaerts drilled a ball right down the left field line. It appeared to soar over the foul pole — making it a home run — but it was called foul. The umpires reviewed the play and the original call was upheld, meaning there wasn’t enough evidence to overturn the initial ruling.

— The Red Sox won a manager’s challenge in the fourth inning.

Mookie Betts made a sensational diving play in left-center field. The initial call was that Betts trapped the baseball, but replays showed he indeed caught it. The play was overturned following a review of one minute, five seconds. The catch saved a run.

— Bogaerts was at the center of a scary scene in the fifth inning.

The 21-year-old was drilled in the head by a pitch from Mariners ace Felix Hernandez. Bogaerts dropped to the ground and stayed on one knee for a minute or so before returning to his feet. He was tended to by manager John Farrell and trainer Rick Jameyson.

Bogaerts initially stayed in the game but was removed before his sixth-inning at-bat to be evaluated for a concussion. The shortstop said he began to lose focus a bit in the field before departing.

Bogaerts didn’t know the results of his concussion tests, but he was in good spirits after the game.

“I feel good right now,” Bogaerts said. “I mean, once I got hit I knew I got hit in a bad place in my head, but I feel good.”

— Yoenis Cespedes crushed a three-run homer in the sixth inning. It was Boston’s only offense.

— David Ortiz (2-for-2, two walks) reached base four times for the fourth straight game.

Ortiz is the first major leaguer to reach base four times in four straight games since Joe Mauer reached base four times in five consecutive games in 2006. He’s the first Red Sox player to accomplish the feat since Mike Andrews in 1970.

— Uehara’s ninth-inning implosion undoubtedly was the worst relief appearance of his career.

He surrendered five runs — all with two outs — on five hits and a walk while suffering the loss. It was the most runs he ever has allowed in a relief appearance. He now has allowed at least one run in three straight appearances, marking the longest such streak of his career as a reliever.

“His last three appearances have had two days off following each one, so we recognize that over the course of two years there’s been a high number of appearances,” Red Sox manager John Farrell said when asked if the club could consider shutting Uehara down for a while. “His workload has been managed of late and we’ll continue to try to do that.”