CLEVELAND — Even hours after his worst start in 15 months, when Rick Porcello had already taken off his uniform, showered, put on street clothes, and could reflect about what happened, confusion still lingered.

How did that just happen? The Cleveland Indians scored about 5.5 runs per game at Progressive Field this year, but Porcello has bested even the most potent offenses during a Cy Young-worthy campaign this year.

Something was off, and even early this morning, the Red Sox were left scratching their heads after Porcello allowed five runs in 41⁄3 innings and stuck the Sox in a hole after taking a 5-4 loss in Game 1 of the American League Division Series.

“Changeup to Francisco Lindor,” Porcello recapped the third of three homers he allowed in the third inning. “I’ve thrown that pitch a lot this year and I haven’t really got hurt by it. Got hurt by it there. You kind of spin it however you want to spin it, but I threw the pitches that I thought were going to be the best pitches to throw and they hit three balls out of the ballpark.”

It was after the second home run that inning, an 0-1 two-seamer up in the zone that Jason Kipnis hammered over the right field wall, that pitching coach Carl Willis thought about doing something.

He stayed in the dugout.

The next batter, Lindor, hit an 0-1 changeup on the outside part of the plate out of the yard. Porcello loves throwing that pitch.

Concerned, Willis ran to the mound.

“It’s like, ‘OK, go to the mound and make sure we are keeping our composure,’ ” Willis explained. “And there is a lot of game left at that point. Told him to just get back to pitch to pitch, executing each pitch.”

Willis added, “I don’t think it was bad by any means, just the times it happened, they seemed to be ready for it.”

It was like the Indians knew what was coming.

Did they?

At some point during the night, the Red Sox switched up their signs, catcher Sandy Leon later told the Herald.

“His signs are unique to begin with, so we talked about mixing it up,” Willis explained. “Just looking for any advantage we can get or to give them something more to think about, if you will.”

Asked if there was any concern that the Indians had figured out Porcello’s signs, Willis smiled and said, “Yeah, you never know.”

It wasn’t the only problem. Porcello couldn’t seem to get ahead of batters all night. He fell behind in counts, and that was his biggest problem, Leon said.

His command also was off. Willis said he thinks the pressure of the playoffs can cause some pitchers to be too fired up on the mound and leak open.

Porcello didn’t have many answers.

“They hit those balls good,” Porcello said. “Two of the three I didn’t think were going to go out. They did. It is what it is. They went out of the ballpark. That’s the bottom line. I have to do a better job of eliminating that.”

By the fifth inning, when Porcello had given away two leads and the Red Sox were down, 4-3, Porcello allowed another baserunner with one out and manager John Farrell had seen enough. Drew Pomeranz replaced him and allowed the inherited runner to score.

It wasn’t only Porcello’s fault, of course.

Mookie Betts, Xander Bogaerts and Jackie Bradley Jr. — cogs of the Sox offense all season — combined to go hitless in 12 trips to the plate.

The Sox rarely win when they can’t score five runs. They were 78-15 when they reached the magic number in the regular season, but only 15-54 otherwise.

The game didn’t start poorly for the Sox.

Dustin Pedroia hit a leadoff double and Brock Holt singled behind him, putting runners on the corners with nobody out. Betts struck out and David Ortiz popped up, but Hanley Ramirez roped a double to left-center. Pedroia scored easily but Holt, originally ruled safe, was thrown out at home, as overturned by a replay review.

Porcello gave the lead away and Andrew Benintendi put the Sox back on top with a solo shot to right off Trevor Bauer in his first career postseason at-bat in the third inning. The 22-year-old became the youngest Red Sox player ever to go deep in a playoff game.

And the 2-1 lead didn’t last long.

Roberto Perez, who hit .183 with three homers this season, started the barrage in the third.

Leon homered in the fifth, but Andrew Miller took over later in the frame and carved up his former team with two scoreless innings.

“Hopefully we win the next three games but if I get an opportunity in the fifth game, you know, I’ll rectify the mistakes I made,” Porcello said.