KANSAS CITY, Mo. — In a season worth flushing, Carlos Rodon added one last mess of a start to go down the drain.

The Yankees left-hander faced eight batters, threw 35 pitches and did not record an out. All eight batters came around to score in a miserable first inning that gave way to the Royals’ 12-5 win over the Yankees on Friday night at Kauffman Stadium.

“It was terrible,” Rodon said.

In the first season of a six-year, $162 million contract with the Yankees (81-79), Rodon finished with a 6.85 ERA across just 64 ¹/₃ innings and 14 starts.

He began the year on the injured list because of forearm and back injuries — with a second IL stint for a hamstring strain in August — and ended it seemingly healthy but ineffective.

“Pretty disappointing,” Rodon said. “There’s not much else to say about it.”

Rodon had been building some late momentum in an otherwise lost season, but it will be difficult to head into the offseason feeling much encouragement after Friday’s clunker.

And it wasn’t just Rodon’s diminished velocity and stuff that raised eyebrows. When pitching coach Matt Blake made a mound visit six batters into the game, with two runners on and the Royals (55-105) up 4-0, Rodon turned his back as Blake was trying to talk to him.

“I was just frustrated,” Rodon said. “I’m sure it was [disrespectful]. I’m sure it wasn’t great. Definitely not the best move. It shouldn’t happen. I was frustrated with myself and my performance. Really embarrassing. Doing that with Matt coming out trying to help me, I turned my back. I was just not in the right mind, that’s for sure. That’s on me.”

Rodon said he wanted to cool off before talking with Blake postgame, and after speaking to reporters, Rodon met with Boone in his office behind closed doors. Boone had said that his initial impression of the mound snafu was that Rodon was being disrespectful to Blake, but wanted to hear more about it.

“Want better mound presence there, but also, the guy’s competing his ass off, and it’s been a tough year for him and he’s trying to figure it out,” Boone said before his meeting with Rodon. “He wants to do well. There’s also some grace in — it’s hard, it’s heavy.

“We don’t have any issue with him as far as how he’s going about it, how he’s getting after it and his care factor. We just got to get him going consistently.”

For the second straight start, Rodon’s velocity was down — including his 25 fastballs averaging 93.6 mph, 1.7 mph below his season average. He insisted he was fine physically but was not sure what was behind the diminished velocity.

Boone pointed to a lack of life on Rodon’s fastball and missed locations as to why the Royals were able to ambush him. But he also said that he could not explain why that was the case Friday night, particularly after Rodon had flashed a more electric fastball in a few of his starts earlier this month.

“There’s no doubt in the talent,” Boone said. “We’ve seen enough this year to know we can get that out. But obviously we got to have a good winter with him and come ready to go in spring training and hopefully have this year be one of those things that throws a log on the fire and motivates you.”

Instead of finishing a rough season on a high note, Rodon went into an important offseason in horrific fashion, clouding exactly what the Yankees have in the lefty heading into next season.

“Just looking forward to taking a little bit of time to myself and getting away from baseball,” Rodon said. “When the time’s right, hopefully in a couple weeks after that, I’ll get back to training and focusing on what I’m going to do on the mound, how I want to move on the mound and find my move and establish that when spring training comes around.”