The Mariners struck first on a fourth-inning solo shot from Jean Segura and would add their second run in the sixth on a Mitch Haniger RBI single, but could not mount a comeback in the defeat.
Starting pitcher Mike Leake allowed the Astros to tie the game in the fifth inning on an RBI double from Josh Reddick. Houston would extend its lead when Evan Gattis took Leake deep with a man aboard in the sixth.
"Probably left a couple pitches over the middle," Leake said. "The Reddick one I didn't put quite where I wanted, obviously it leaked over the middle. And then you have the curveball that just stayed up [to Gattis]."
Two of Seattle's newly acquired relief pitchers made their Mariners debuts in Tuesday's contest, but only one impressed. Sam Tuivailala fired 1 2/3 scoreless innings and stranded his only inherited baserunner to keep the Mariners in a one-run game through the eighth inning.
Zach Duke, however, was brought in for the ninth when the Astros had three straight left-handed hitters due up. Duke allowed a single to Kyle Tucker and a two-run shot to Reddick, which gave Houston a three-run lead.
"Tui threw the ball very well," Servais said, "and Zach Duke hadn't given up a homer to a lefty all year and lo and behold, he throws one down and in to Reddick and he gets it."
Servais also spoke to the importance of getting his new players some work.
"Any time guys join a new club they want to contribute right away," Servais said. "You don't really feel like you're a part of it until you get out there. I was anxious to see both those guys throw."
Houston now leads Seattle in the American League West by four games. Meanwhile, an Athletics victory over the Blue Jays means the Mariners are holding onto the second spot in the AL Wild Card by one game.
SOUND SMART The Mariners were baseball's least prolific offense in the month of July, scoring just 77 runs.
HE SAID IT "Houston's got good pitching. That's the best pitching in the American League. There's a reason they have the record they have, but you gotta continue to figure out a way, try to create some more traffic on the bases." -- Servais, on the Astros' pitching and effectiveness of starting pitcher Charlie Morton
MITEL REPLAY OF THE DAY Dee Gordon was picked off of first base for the third out of the fourth inning -- a call that stood following the review.
Although Gordon made an out, he also got Segura a fresh count. Segura was down 1-2 when the inning ended. He ended up homering when he led off the next inning on a 0-1 pitch.
UP NEXT The series will close out at 1:10 p.m. PT with a matchup between lefties Wade LeBlanc (6-1, 3.51 ERA) and Dallas Keuchel (8-9, 3.60). LeBlanc gave up three runs and lasted just four innings in his prior start against the Astros. In Keuchel's last start at Safeco Field, the Mariners tagged him for seven earned runs in 6 2/3 innings pitched.