They were already there at the onset of this season's second half, but a 6-0 loss to the Red Sox on Sunday meant a three-game sweep to start this stretch.
Kansas City (48-49) has dropped 9 of their last 12 and fall back below .500 for the first time since June 8. Their next 10 games come against American League Central opponents.
Yordano Ventura (7-8) gave up six runs on nine hits over 4 1/3 innings, recording no strikeouts. Dustin Pedroia began the scoring in the first inning with an infield grounder that Alcides Escobar couldn't get out of his glove.
Daniel Nava plated two more in the fourth with a two-run ground rule double that snuck inside the right field foul pole. He finished with a game-high three RBIs, while David Ross added his sixth home run in the 4th.
Jone Lester (10-7) continued his success against the Royals, striking out eight over eight innings while allowing four hits.