Seattle (45-57) has eight wins in its past 12 games in part because it's gotten plenty of chances to face Kansas City. Since July 15, the Mariners have won five of six against the Royals (41-58).
Jaso is batting .345 over his last 21 games and has the best average among Mariners regulars at .288.
"One thing you know you're going to get from him is a quality at-bat," manager Eric Wedge said. "If you look at his at-bats, that's what we're looking to do with all of our guys."
The Royals, meanwhile, have scored a combined two runs in this series and Billy Butler has four of his team's six hits. He had two singles during Kansas City's 19th loss in 25 games and 12th in 15 road contests.
"We just haven't been swinging the bats lately," manager Ned Yost said. "You can't expect to be successful every night when you are down four, five, or six runs in the first two innings."
Millwood (3-8, 4.13 ERA) is hoping to get similar support from the Mariners in this game and earn his first win since May 23. The veteran is 0-4 with a 4.56 ERA over those 10 starts and has received a combined 19 runs of support.
He took the loss in a 4-1 defeat to the New York Yankees on Monday after allowing three runs over seven innings.
To end his drought, Millwood will likely need a better performance than in Kansas City on July 18. He gave up seven runs and a season-high 10 hits over five innings during Seattle's 8-7 loss.
The opposing pitcher for that game was Chen (7-8, 5.54), who surrendered four runs in 5 1-3 innings while not factoring in the decision. The left-hander is 0-2 with a 9.25 ERA over his last five starts but had one of his best performances in that stretch Monday on the road against the Los Angeles Angels, allowing three runs and five hits over 5 1-3 innings.
"I thought I made good pitches, I battled," he told the team's website.
Chen is 4-0 with a 2.38 ERA in eight career starts against the Mariners and 2-0 with a 1.31 ERA over three in Seattle. The most recent of those visits was on Aug. 7, 2010.
Millwood is 0-2 with a 5.16 ERA in his last five matchups with Kansas City.