Chen had won five straight starts before losing his last two, including an awful outing on Wednesday in Oakland. He gave up six runs (five earned) and eight hits with three walks in 5 1/3 innings of that one and fell to 10-7 to go along with a 4.30 earned run average.
The Panamanian lefty was brilliant in beating the Mariners the last time he faced them, allowing just an unearned run in eight innings, and is a perfect 4-0 in 11 games (seven starts) against them with a 2.73 ERA.
Seattle will counter with young lefty Anthony Vasquez, who has lost his last two starts after winning his big league debut back on August 23. Vasquez lost to the Angels on Monday, surrendering seven runs, just three of which were earned, and five hits in 5 2/3 innings to up his ERA to 9.00.
This will be his first-ever start against the Royals.
On Saturday, Eric Hosmer's two-run homer in the first inning proved to be the difference, as Kansas City prevailed, 4-2. Jeff Francoeur hit a solo homer for the Royals, who snapped a three-game skid.
Kansas City starter Felipe Paulino (3-6) gave up two runs on two hits to go with a season-high 11 strikeouts over seven innings.
Rookie Michael Pineda (9-10) allowed three runs on five hits over a career- high eight innings to take the loss. He struck out eight and walked one.
"That was pretty quick wasn't it? That's what happens when you get two overpowering guys matched up against each other; it's normally going to be a quick game," Royals manager Ned Yost said.
Kansas City has won four of its seven meetings with the Mariners this season. However, Seattle is 31-20 against the Royals at Safeco Field, which opened in 1999 -- its second-best winning percentage against an opponent there.