Earlier Monday, the Mariners designated veteran outfielder Ryan Langerhans for assignment and recalled outfielder Mike Wilson from Tacoma.
Both Peguero and the right-handed-hitting Wilson will be with the team Tuesday when it opens a three-game series at Baltimore. The Mariners have won eight of their past 12 games to improve to 16-19, but scored just 18 runs in their last eight games.
Zduriencik said Peguero and Wilson were "both guys who have a chance to be part of the future and both power hitters," and said the two would be used as a left-right combination in left field.
Bradley's time with Seattle, much like the rest of his Major League career, was tumultuous. He was ejected from two games in the past 10 days and sat out another on a one-game suspension. The 32-year-old has declined to speak with reporters most of this season and wore earplugs during many of his games, apparently to keep out the taunts of fans both at home and on the road.
A year ago, he left Safeco Field in the middle of a game after being taken out by then-manager Don Wakamatsu, then was placed on MLB's restricted list while undergoing counseling. He rejoined the team, but spent the last two months of the season on the disabled list with a knee injury.
New manager Eric Wedge put Bradley in the middle of his lineup in the No. 3 hole and started him in left field in 26 of the team's first 35 games. But after a good start at the plate, he hit just .159 (7-for-44) with three RBIs and no home runs over the last three weeks as his batting average fell to .218.
Additionally, Bradley struggled defensively in left field and his errant throw on a play at the plate Sunday wound up leading to two runs scoring when catcher Miguel Olivo dropped the ball and was unable to make the tag in a 3-2 loss to the White Sox.
Zduriencik said he and Wedge were in full agreement on the decision to part ways with Bradley, but that there was no single incident that led to the decision.
"I don't think there's any particular instance that gets you to this point," Zduriencik said. "There can be, but not in this case. In this situation, we evaluated where we are and where we're going, and in our estimation, he did not fit."
Bradley had 10 home runs and 42 RBIs in 345 at-bats over two seasons in Seattle after being acquired by trade with the Cubs for pitcher Carlos Silva. He is still owed the remainder of a $12 million contract for this season.
Zduriencik noted that trade was made with the hope of adding offense a year ago after it was determined Silva wasn't going to be part of the 2010 team.
"That was a situation where we didn't think Carlos would be part of the future. And obviously Milton wasn't part of their future in Chicago," he said. "We were trading, in essence, contract for contract and hoping it worked for both clubs.
"We were in pursuit of a bat and had dollars committed to a contract. We were hoping it would work for both clubs. At this point, it hasn't worked out for either club the way we were hoping."
Silva was released by the Cubs this spring. The Mariners now have 10 days to trade, release or outright the contracts of both Bradley and Langerhans.
While Wilson will be making his Major League debut with the Mariners, Peguero got his feet wet briefly from April 19-24, hitting .183 (2-for-11) with five strikeouts filling Smoak's roster spot.
The Dominican Republic native has hit .282 with four home runs and 13 RBIs, sporting an .801 OPS in 103 at-bats with Tacoma this season.
Wilson, 27, has been Tacoma's hottest hitter with a .381 average and four home runs and 14 RBIs and a 1.111 OPS in 63 at-bats after missing several weeks to start the year with a rib injury.
The Mariners are still awaiting the return of veteran outfielder Franklin Gutierrez, who remains on the 15-day disabled list as he returns from stomach issues that shut him down midway through Spring Training. Gutierrez is on a rehab assignment with the Rainiers, but didn't play Sunday after coming down sick with an unrelated flu-like illness.