Brad Peacock had been solid for the Houston Astros since returning from Triple-A early this month.
He couldn't continue that success on Friday night against Seattle and the Astros lost 7-1 to the Mariners.
Peacock (3-5) had a 2.88 ERA with 25 strikeouts in his previous four starts this month, but was tagged for seven hits and five runs in 4 2-3 innings.
''I felt like I had made great progress until I had this rough one, but I'll move on from it and be ready for that next one,'' Peacock said. ''I've still got five starts left this year. I think there's a lot of young guys up here, and we're all trying to make names for ourselves.''
Top prospect Taijuan Walker did not allow an earned run over five innings in his major league debut for Seattle and Dustin Ackley had a career-high four hits with four RBIs.
The Astros were up by one in the fourth before Ackley tied it with an RBI single. Seattle led by two in the fifth before a single by Ackley made it 5-1. A run-scoring single in the ninth gave Ackley four RBIs to tie a career best.
Houston manager Bo Porter was disappointed that Peacock took a step back on Friday.
''There were just too many deep counts,'' Porter said. ''It's tough when you give guys that many pitches, especially guys that feast on the fastball. He just didn't have his offspeed stuff working tonight. They forced him into some fastball counts and put good swings on him.''
''Today was probably the worst start he's had since he came back up,'' Porter continued. ''And it was mostly about his secondary pitches.''
Walker, a 21-year-old right-hander who is rated the fifth-best prospect in baseball, yielded two hits and one run with two strikeouts and a walk. If the 6-foot-4 Walker (1-0) had any rookie nerves, it certainly didn't show as he retired the first eight batters he faced.
Abraham Almonte, also playing his first major league game, added two RBIs to help the Mariners win their second straight following a six-game slide.
Seattle relievers Carter Capps, Charlie Furbush and Oliver Perez combined to pitch four scoreless innings after Walker left the game.
Walker relied heavily on a fastball that was routinely clocked at 95 mph, but reached as high as 97 mph. He also used a cutter, curveball and slider to keep the Astros off balance.
''He was really impressive,'' Houston's L.J. Hoes said. ''He's a big kid. On his video, he didn't look as big as he really is. It says he's throwing 96 (mph), but it seems way harder than that because he's so big.''
A first-round draft pick in 2010, Walker joined the Mariners after going 4-7 with a 2.46 ERA in 14 starts at Double-A and 5-3 with a 3.61 ERA over 11 starts in Triple-A. He was dominant in his last two outings, going 2-0 with a sparkling 0.82 ERA.
Walker became the youngest starting pitcher to make his debut for the Mariners since Felix Hernandez's first game in 2005 at 19. The Mariners offset Walker's inexperience by starting catcher Henry Blanco, who turned 42 on Thursday. Manager Eric Wedge thought Blanco's presence would help make Walker feel more comfortable in his first game.
He was rolling before a tough third inning for Seattle's defense. Houston's first hit came with two outs on a double by Jonathan Villar that bounced off the glove of left fielder Raul Ibanez.
An error by first baseman Justin Smoak allowed Robbie Grossman to reach safely as Villar scored to give Houston a 1-0 lead.
L.J. Hoes singled on a grounder that deflected off Kyle Seager's glove at third base. But Walker didn't get rattled, and after a short visit to the mound from Blanco, he retired Jose Altuve to limit the damage.
Walker set down six of his last seven batters, with the only baserunner coming on a walk to Brett Wallace with two outs in the fourth.
Walker was helped by some dazzling defense for the last out of the fifth. Ackley sprinted before sliding on his knees to grab a fly ball hit by Grossman near the bullpen in right-center. Walker pumped his fist and smiled before leaving the mound.
Kendrys Morales walked to start the fourth and Smoak drew a one-out walk. The Mariners tied it on an RBI single by Ackley before taking the lead when Almonte grounded into a forceout that scored Smoak.
Brad Miller singled to start the fifth and stole second. Seattle pushed the lead to 3-1 on an RBI single by Seager.
He stole second and advanced to third on a wild pitch. Ackley's two-run single to center extended the lead to 5-1.
NOTES: Seattle LHP Joe Saunders (10-13) opposes Houston's Dallas Keuchel (5-7) on Saturday. ... Houston C Carlos Corporan, on the seven-day concussion list, said he no longer has concussion symptoms and is ''back to himself.'' He has been doing individual work, but isn't sure when he'll return. ... Olympic gold medalist and softball star Cat Osterman threw out the ceremonial first pitch.