KANSAS CITY — Chalk this one up as another learning experience for Royals rookie right-hander Brad Keller.

Keller clearly didn’t have his best stuff or command on Tuesday against the Cubs, but he reached deep and grinded out six serviceable innings in a 5-0 loss at Kauffman Stadium.

The Royals have lost six straight.

The Cubs jumped on Keller in the first inning, beating him twice on sliders and once on a changeup as Javier Baez singled, Ben Zobrist doubled and David Bote tripled for a 2-0 lead.

“[Keller] kept us in the game,” manager Ned Yost said. “He did a good job of doing that. He made pitches and limited the damage when he needed to. Command was off, the evidence was three walks [later]. But it was off in the first inning, too, on the plate. He was missing quality spots and center-cutting some pitches that they took advantage of.

“I thought he was gonna get out of the first inning scoreless, but kind of threw a slider that Bote hit off the wall in right field for the two runs. But did a good job of regrouping there and giving us an opportunity to win.”

Keller, though, then began leaning on his four-seam fastball and tossed three scoreless innings, though he walked three in that span.

“Just two pitches in that first inning — changeup and a slider were off, and they did some damage,” Keller said. “Besides that, I thought the fastball was there and put it where I wanted it.”

The Cubs nicked Keller for two more runs in the fifth on four singles sandwiched around a sacrifice fly. Keller gave up seven hits and four runs over six innings. He struck out four.

“That fifth inning snowballed on me,” Keller said. “I made good pitches and balls fell in. That’s baseball.”

Meanwhile, the Royals continued to struggle offensively, this time against former farmhand Mike Montgomery, who threw six-plus scoreless innings. They now have lost 10 straight games when the opponent starts a left-handed pitcher.