An undrafted free agent from the Dominican Republic, Pineda made his big- league debut with a loss at Texas on April 5, but proceeded to win his next four starts and six of his next seven decisions.
He's tailed off badly since, winning just three times since mid-June and not at all since July 30, when he beat Tampa Bay, 3-2, behind 6 1/3 innings of one-hit ball.
Two September starts have ended in losses, the most recent being a 4-2 decision to Kansas City in which he gave up three runs on five hits in eight innings on Sept. 10
Pineda won his lone outing against the Twins on May 16 in Seattle with seven innings of scoreless, three-hit ball in the Mariners' 5-2 victory.
On Tuesday, Adam Kennedy's run-scoring infield hit in the seventh inning was the difference as Seattle sent Minnesota to its 10th consecutive loss with a 5-4 decision.
Alex Liddi hit a two-run game-tying homer and Mike Carp totaled a career-high five hits -- including an RBI double -- for the Mariners, who have won four of their last six.
Ben Revere had four hits and an RBI for the Twins, who hadn't seen a double- digit losing skid since dropping 10 straight in September of 1998. Trevor Plouffe doubled twice and drove in a run in the setback.
Brian Duensing (9-14) served up the deciding run in 1 1/3 relief frames.
The Twins start righty Kevin Slowey, who'll make his 13th attempt at a first victory of 2011.
The 27-year-old Texan won 13 times in 30 appearances last season, but struggled from the outset in 2011 in a combined relief/starter role.
He's started six games -- and lost all six -- since Aug. 19, most recently a 7-6 verdict against Cleveland in which he allowed five runs on seven hits in four innings.
Slowey hasn't faced Seattle this season, but is 2-1 against the Mariners in four career starts with a 3.51 earned run average in 25 2/3 innings.
The Mariners have won four of six versus the Twins this year.