Mazzaro, 22, hasn't gotten past the sixth inning in each of his past 15 starts, and he will be replaced as Friday's starter by rookie righty Clayton Mortensen, who will be called up from Triple-A Sacramento for his second spot start of the season.
"He hasn't bounced back from start to start as well as we'd like," Geren said of Mazzaro.
The A's bounced back nicely Thursday after Tomko put them in a big hole; he allowed no-doubt homers to Franklin Gutierrez and Bill Hall in the top of the first inning and trailed, 4-0, after 16 pitches.
Oakland's offense struck right back with a two-run single by Kurt Suzuki in the bottom half of the frame, and Seattle's lead was cut to one when Mariners shortstop Jack Wilson made an errant flip to second base with two out in the bottom of the second, allowing Adam Kennedy to score from second base.
Hall, however, got the visitors going again with a leadoff single in the fourth, and Kenji Johjima's homer with one out was the dagger that pierced Tomko's perfect record since joining Oakland in mid-August.
Tomko, who beat the Yankees in his first start, topped the Tigers in his second and left his third with a five-run lead against the Angels, saw his ERA with the A's jump from 1.62 to 4.40 in 5 2/3 innings.
"I fell behind all three guys," Tomko explained, "and when you fall behind guys and don't make quality pitches ... that's exactly what happens. I got too much of the plate. They're looking to drive the ball, and I played right into their hands."
"He just got hurt up on some pitches up in the zone," said Seattle skipper Don Wakamatsu, whose club is 10-3 against the team for whom he served as bench coach last season. "He's made many improvements, though. I've talked with some of his coaches, and he's making adjustments. But it's all about location, location, location."
Tomko's location on the three fateful pitches was low-rent.
"All three were right out over the plate," he copped. "I wish I could take those three pitches back."
Tomko allowed eight hits -- five for extra bases -- and walked one while striking out two, while Seattle starter Ian Snell picked up his fourth victory in five decisions since being acquired in a trade with the Pirates on July 29.
Snell wasn't dominant by any stretch, allowing three runs (two earned) on seven hits and three walks over five innings.
"They're a pretty patient bunch of hitters," Snell said. "I was trying to throw them fastballs down the plate, but it was moving too much. My sinker was out of control, and so was my changeup."
Yet Snell seemed perfectly in control amid constant traffic on the bases, leaving runners at the corners in the second inning and stranding a pair of runners in the fourth and fifth frames.
"We had a lot of opportunities [against Snell], myself included," said A's second baseman Mark Ellis, who went 0-for-4 with three strikeouts, including two with men on base. "We just didn't get it done."
The Mariners' bullpen ERA of 3.74 entering the game was the main reason for the pitching staff's status as the American League's stingiest, and just as they did during a three-game sweep in Seattle last week, the relievers ripped through Oakland's lineup in a hurry.
Daric Barton's solo homer in the eighth was one of two hits for the A's over four innings against Chris Jakubauskas, Mark Lowe and David Aardsma, with Aardsma picking up his 34th save.
"We had a lot of chances early," Geren said, "but after that, [the] relievers pretty much shut us down."
Added Ellis: "They have some really good arms down in the bullpen, so you definitely want to get to them early."
Mortensen, 24, was acquired in the July trade that sent Matt Holliday to the Cardinals, and he made his A's debut in early August at Kansas City, allowing eight runs on eight hits over four innings.
Mazzaro might not pitch again until next Friday at Texas.
"He won't start before the off-day [Monday], for sure," Geren said.