The 24-year-old right-hander gave up a run in the first, five in the second and another in the third to the youthful Indians' lineup before giving way to Aaron Looper.
Looper faired little better, giving up three earned runs in 2.1 innings, including a three-run homer to Travis Hafner. That put the Tribe up 10-2 at that point.
Seattle stormed back to close the gap to 10-6 behind John Olerud's three-run homer, but that's as close as the M's would come.
Bret Boone also homered for the M's, his 29th on the season.
The Mariners (69-44) lost a game of their American League West lead over Oakland. The Athletics won Wednesday to bring the M's lead back to three games.
Cleveland (46-67) powered out 15 hits en route to their 10 runs. Leadoff hitter Coco Crisp led the charge with a 3-5 day.
Crisp led off the game with his first homer of the year.
Pineiro dropped to 13-6 and saw his ERA rise from 3.03 to 3.38 in the process.
Indians' starter Jake Westbrook (5-6) gave up six runs in 5.2 innings, but came away with the victory.