ST. PETERSBURG -- When Rays manager Kevin Cash and head athletic trainer Joe Benge made their way to the mound in the fourth inning Thursday night, Shane McClanahan made his case to stay in the game.

But with his back tight, velocity down and extension limited, McClanahan knew the right thing to do. Tampa Bay’s ace walked off the mound alongside Benge, and his early exit was one of many frustrating moments in the Rays’ 6-5 loss to the Royals at Tropicana Field. It was only the second time this season they have lost when leading after seven innings.

McClanahan was officially diagnosed with mid-back tightness. Cash said the Rays will know more about his status in the coming days, after he receives treatment on his back, but McClanahan is optimistic it won’t be an ongoing issue.

“I don't want to speculate. It's still pretty early, but the leading notion is we don't think it's anything severe,” McClanahan said. “Just maybe more of a precautionary move just to maybe limit whatever it is. Hopefully, I'm not missing any starts, and we get this thing hammered out in a couple of days.”

McClanahan’s fastball averaged 95.1 mph on Thursday, down from his season average of 96.7 mph. He displayed his typical velocity early on, running his heater up to 97.2 mph in the first inning and throwing a handful around 96 mph in the third. 

But his last fastball to Dairon Blanco clocked in at 93.6 mph, and he threw a four-seamer to Drew Waters earlier in the inning that registered at 90.9 mph, according to Statcast. He only topped 94 mph once in the fourth.

That was enough of a warning sign for Cash and pitching coach Kyle Snyder to warm up reliever Kevin Kelly in a hurry, and Kelly was busy on the bullpen mound as McClanahan retired Blanco for the second out of the fourth. Cash and Benge jogged out of the dugout to visit McClanahan, who wanted to keep pitching but eventually gave way to Kelly.

McClanahan said he felt “a little tightness” leading up to his last start, but it subsided by the time he took the mound in San Diego. He couldn’t shake it on Thursday, however, and he acknowledged he might have adjusted his delivery in a way that diminished his stuff.

“Pretty frustrated that I had to come out of the game. I understand the decision,” McClanahan said. “I felt like I could have kept going, but it is what it is. It's up to myself and the training staff now to try to get this thing right.”

Last year’s All-Star Game starter for the American League, McClanahan has been one of the best starting pitchers in the Majors this season. He owns baseball’s highest win total (11) and lowest ERA (2.23) to go along with 100 strikeouts in 93 innings.

“Appreciate him, certainly, wanting to go out there and to continue to compete,” Cash said. “But I think in those moments, we probably need to err on the side of caution and make sure that we do right by him.”

Waters put the Royals ahead, 3-2, with a seventh-inning homer off Rays reliever Robert Stephenson. Tampa Bay answered with three runs of its own via a Francisco Mejía home run and a go-ahead, two-run single by Randy Arozarena.

The Royals made it a 5-5 game in the eighth on Waters’ two-run single off Jason Adam. With top high-leverage reliever Pete Fairbanks pitching the ninth, Kansas City walked and ran its way to the winning run.

Maikel Garcia worked a leadoff walk then immediately stole second and third -- two of the Royals’ seven steals on the night, tying the record for the most allowed by the Rays in one game. Fairbanks credited Royals bench coach Paul Hoover, a former Rays coach, for identifying the opportunity to steal against him.

“I know that we have big swing-and-miss stuff there at the back end of the bullpen, but we put ourselves a little bit behind by giving those extra 90 feet,” Cash said.

Fairbanks struck out the next two hitters and got MJ Melendez to hit a 71.9 mph grounder toward first base. But the ball rolled into a tricky spot. Fairbanks covered the bag while first baseman Yandy Díaz went for the ball, and Díaz’s late flip landed behind Fairbanks as Melendez reached safely and Garcia scored.

“It's a frustrating way to lose a ballgame, but we'll get back at 'em tomorrow,” Fairbanks said. “It is what it is. It wasn't very well hit. It was in the right spot, and that cost us.”