Billy Butler swung a pair of Mariucci bats inside the visitors’ clubhouse at Fenway Park, rotating the lumber from hand to hand, pondering the factors behind his first-half torment.

“Some years, you just have a down year,” said Butler, the Royals’ designated hitter. “And you feel the same. For whatever reason, it just happens. I’m not saying you’re any less of a player than you were.

“My bat speed’s still there. I get to certain pitches. I mean, these shifts are messing with me, too. I hit balls up the middle, and they get to them. They used to be hits. When I came up, there were no shifts!”

Butler was smiling. The worst offensive season of his career has not dented his confidence, and has not blemished his faith in his ability. He understands the Royals face a decision on his contract after this season, with a $12.5 million team option that appears unlikely to be picked up.

But in Butler’s view, when it comes to that option, his .811 career on-base plus slugging percentage should outweigh his .679 OPS in 2014.

“My numbers throughout my contract suggest they should (pick it up),” Butler said. “Maybe not the way I’ve necessarily played this year, but that’s the reason you play a full 162 games. The evaluation is there. That’s the reason why you have a track record.

“If you want to go off 90-something games, you can say, ‘yeah, it’s not justified.’ If you want to go off of other ones, yeah, it probably is. It’s all how you want to look at it.”

At 28, Butler continued, “my best years are still ahead of me,” and he insisted he would like to spend the rest of his career as a Royal. Which is why, if Kansas City does not intend to pick up the option, he said he would be willing to renegotiate an extension at a lower rate than $12.5 million per season.