BOSTON — Perhaps the Chicago Cubs are a good come-from-behind team because they’re getting so much practice at it.

They’re now 4-6 when trailing entering the sixth inning after a 5-4 loss to the Boston Red Sox on Friday night — which means in 10 of 22 games this season, they’ve been behind the 8-ball heading into the late innings.

For the second straight start, Jake Arrieta was handed a first-inning lead only to give it right back as the Red Sox scored all five of their runs in the bottom of the first inning. The Cubs inched their way back but never got the big hit to put them over the top.

“We got beat today based on their output in the first inning,” Arrieta said. “That was on me.”

Make that nine runs given up by Arrieta in the first inning of his past two starts. And for the season, the Cubs have a 9.82 first-inning ERA, the worst mark in all of baseball. Yes, it has been that bad to start the game. Last season, they were first in the National League and second in baseball in the same category.

“We play that game almost every night,” manager Joe Maddon said of starting from behind. “I am certain that is going to go away. Like I said, [the Cubs starters’] stuff is good. I don’t have any strong explanation as to why. I really believe it’s going to trend in the right direction.”

In fact, no one in the Cubs’ tiny clubhouse at Fenway Park could pinpoint why they’re giving up so many first-inning runs. On Friday, after getting the first batter on a ground ball, Arrieta allowed the next seven to reach, including an Andrew Benintendi homer. The Cubs lead the league in first-inning home runs given up as well. That was No. 8 in game No. 22.

“You’ll see a lot of strange things in this game,” Arrieta said. “You’ll see streaks that are very impressive and you’ll see them end like they did tonight or the last couple of starts.”

Before this week, Arrieta had not given up four or more runs in back-to-back first innings in his career. But now he has, and the Cubs simply can’t put their collective fingers on why it’s happening.

“I’d have to really look at it to see if it plays together,” catching coach and game-planner Mike Borzello said.

The Cubs are without their pitching coach this week as Chris Bosio attends to personal matters, but that’s no reason for the early-inning woes. It’s been a month-long trend, and if it becomes a pattern, it won’t bode well long-term. Even a partly partisan Cubs crowd couldn’t cheer the visitors on to overcoming that early deficit. It doesn’t help when they strand 10 runners and go 1-for-11 with them in scoring position. But it’s that first inning that will keep Arrieta up at night.

“It’s not ideal,” Arrieta said about getting down early. “At the end of the day, it wasn’t my best.”

The strange thing is Arrieta settled in after the first inning as he got away from his cutter and went to more changeups and curveballs, but the damage was done. Perhaps this is all an easy fix as Cubs starters just need to bear down more in the first inning — whatever that means. Or, maybe they need to adjust quicker. Jon Lester, for example, likes to feel out his opponent and will switch game plans, but sometimes that’s after the damage was done. Same could apply to Arrieta, though sometimes it’s easier to see it after the fact then during it. Just ask the coaches who are watching it all play out.

“You don’t know you need to adjust until it’s too late sometimes,” Borzello said.

Friday was one of those times, and it helped cost the Cubs a game.