It became clear early in the game that Morrow, who entered the start with a 3-0 record and 1.89 ERA in three career outings against the Mariners, wasn't his sharpest.
He needed 27 pitches to get out of the first inning and put the Blue Jays in an early hole, allowing three consecutive hits to start the frame. Morrow later surrendered a two-out single to Michael Saunders that put the Mariners ahead, 2-0.
"They were just attacking early ... did a good job of putting the bat on the ball there, in that inning," said Morrow, who fell to 8-6 and saw his ERA rise to 3.16.
Morrow, who said he felt out of sync with his body and had trouble finding his release point, believes the only reason he was able to limit the damage was the effectiveness of his changeup. He was also bailed out by Rajai Davis on a pair of occasions early on.
Davis threw out John Jaso at the plate to end the first and then nailed Jaso again at third base in the third inning with the Mariners up, 3-1, for his second outfield assist of the game and eighth of the season.
"He [Davis] has done an excellent job in terms of making more accurate throws and getting better carry on it," manager John Farrell said. "It's a credit to the early work and the extra work that he does and he has made himself into a solid left fielder defensively."
Morrow, who had already thrown 80 pitches after four innings, surrendered a leadoff homer in the fifth to Kyle Seager, his 18th of the season, before Saunders knocked him out of the game by hitting a two-out ground-rule double to right-center field.
"It was just one of those days," said Morrow, who lasted 4 2/3 innings, walked one and struck out four. "You just kind of feel like it's not your day."
Morrow was constantly working from behind, as he threw first-pitch strikes to just 11 of the 24 batters he faced -- a far cry from his previous outings.
In his first three starts since returning from the disabled list with an oblique strain, Morrow went 1-1 with a 2.60 ERA over 17 1/3 innings. Tuesday's loss was the first time Morrow has allowed more than two earned runs in a start since May 25 and the third time in his career he has given up 10-plus hits.
"He wasn't as powerful, wasn't as sharp. It was a little bit more difficult for him tonight to command the bottom of the strike zone," Farrell said.
The Mariners pounded out 18 hits and had five players record multihit games, including Jesus Montero, who extended his hitting streak to a career-high nine games.
"I really liked the way we swung the bats tonight. That's more indicative of where we need to be and what these guys are capable of doing, so that was great to see," Mariners manager Eric Wedge said.
"Morrow has given us fits. He's been so good against us. So to do it against him, all the more, because he has great stuff."
Toronto, meanwhile, had trouble solving Mariners starter Erasmo Ramirez, who made his first start since June 30. The rookie lasted seven innings, allowing two runs on six hits while walking one and striking out six to earn his first Major League victory.
Once Ramirez was out of the game, Toronto made things interesting.
Down 4-2 in the eighth, the Blue Jays added a run after Colby Rasmus, who led off the frame with a ground-rule double to right field, scored on a groundout by pinch-hitter Moises Sierra to make it a one-run game. Mariners reliever Stephen Pryor got Yunel Escobar, who went 2-for-4 with an RBI, to ground out to end the inning.
The Blue Jays, however, continued to fight in the ninth. Davis reached on an infield single with two outs and stole his 42nd base of the season to put the tying run in scoring position for Anthony Gose. But Gose grounded out to short to end of the game and sent the Blue Jays to their fourth consecutive loss against the Mariners.
Former Blue Jays outfielder Eric Thames made his return to Toronto since being traded to the Mariners at the non-waiver Trade Deadline and went 0-for-4 with three strikeouts.
Brett Cecil, Brad Lincoln and Jason Frasor combined to throw 4 1/3 scoreless innings of relief.
With the loss, the Blue Jays lost their first season series to Seattle since 2008 and fell to 11-22 in one-run games.