KANSAS CITY, Mo. — Clint Frazier began a rehab assignment with Low-A Tampa on Tuesday as he tries to come back from the mystery ailment that has sidelined him since he left a game on June 30.

Manager Aaron Boone said the Yankees have been unable to determine exactly what has caused Frazier’s issues, testing for concussion-related symptoms, as well as vision issues.

“I know he’s in a pretty good place,’’ Boone said before Frazier went 2-for-4 with a double and two strikeouts for Tampa.

But when asked if there is even a spot for Frazier on the roster now that the Yankees have added Joey Gallo, Boone didn’t say yes.

“We’ll see,’’ Boone said. “That’s a ways off to even think about. … I’m not gonna speculate. Our roster is as fluid as it could possibly be. Every time I’m making a plan for the next few days, something gets turned upside down.”

He did note, “We know what Clint is capable [of] and his talent. He needs to get games under his belt.”

Giancarlo Stanton started in left field Tuesday in an 8-4 loss to the Royals at Kauffman Stadium.

Gallo went hitless and struck out three times in the Yankees’ loss as his struggles continue. Since a three-hit game on Aug. 5, Gallo is 2-for-18 with 11 strikeouts.

Boone remains unconcerned.

“He’s gonna strike out,’’ Boone said. “Along with it is a guy, over time, who’s gonna get on a ton, hit with authority and be a great two-way player for us.”

Despite the lack of results, Boone said Gallo is “in the fight all the time. There’s gonna be a stretch when he’s gonna get super hot.”

Luis Severino (elbow surgery, strained groin) is scheduled to make another rehab start Friday and could be back with the Yankees next week, while Corey Kluber is due to make his first rehab start from a strained shoulder on Thursday.

Domingo German (shoulder inflammation) could throw a bullpen session Thursday.

There’s no help coming from Deivi Garcia, who allowed five earned runs in 4 ¹/₃ innings Tuesday for Triple-A Scranton/Wilkes-Barre. The struggling right-hander also walked five as his ERA jumped to 7.25.

Boone said there was no update on Aroldis Chapman, on the IL with left elbow inflammation. … Not generally known for their speed or prowess on the basepaths, are tied for the MLB-lead with 20 stolen bases since the All-Star break.

The Yankees have already used 57 players this season, their second-highest total in history (58 in 2014). … Andrew Velazquez started at shortstop for a second straight game after being promoted from Triple-A SWB on Monday. The Bronx native got his first hit as a Yankee with a third-inning double and scored later in the inning.