"Our strengths and their strengths and surplus really lined up well," said Dipoto, who indicated the two teams have been talking since the end of the regular season. "The two things the Rays had quite a bit of was Major League-ready starting pitching and athletic outfielders throughout their system.
"Karns enters our rotation immediately and is a very strong upgrade for us in that regard. He's a power arm with power stuff coming off a really solid first season. And in 2016, at some point, whether it's April or later in the year, Boog Powell will become a top-of-the-order, athletic, on-base center-field option that should give us a lot of return for years to come. And that's exciting."
Woke up a @RaysBaseball , going to bed a @Mariners ! Thank you for all the support, excited for the next chapter!
Karns, 27, was 7-5 with a 3.67 ERA in 27 games as a rookie with the Rays last season. The 6-foot-3, 225-pound Texas native led all American League rookies in innings pitched (147), strikeouts (145) and starts (26), while holding opponents to a .239 batting average before being shut down in the season's final two weeks with some minor soreness in his forearm.
Powell, 22, was ranked as the No. 13 prospect in the Rays organization by MLBPipeline.com. He hit .295 with 16 doubles, nine triples, three homers and 18 stolen bases between Double-A Montgomery and Triple-A Durham in 2015. He was named to the Southern League All-Star team with Montgomery after hitting .328 with a .408 on-base percentage in 61 games, then batted .257 with 10 doubles, three triples and two home runs in 56 games with Durham.
Powell was obtained by the Rays in a trade with the A's last year. He's a left-handed hitter who can play center field and has posted a low strikeout and high walk rate in the Minors, where he has hit .308 with a .401 OBP and 53 stolen bases in 294 games over four seasons. His official name is Herschel Mack Powell IV, but he has gone by Boog Powell since his grandfather referred to him by the name of the former Orioles slugger during his childhood.
With Miller's departure, the Mariners' only returning center-field candidates are September callups James Jones and utility man Shawn O'Malley, and Dipoto said Powell will come to Spring Training with a chance to compete for a spot.
Riefenhauser is a "nice, flexible bullpen option," according to Dipoto. The 25-year-old made 17 appearances and posted a 1-0 record and 5.52 ERA over four stints with the Rays in 2015. In his final 11 appearances as a September callup, the left-hander had a 2.16 ERA in 8 1/3 innings and closed out the year with seven straight scoreless outings.
The Mariners gave up three players who were part of their Major League club the last two seasons. Morrison, 28, appeared in 146 games last year while hitting .225 with 17 homers and 54 RBIs, but he lost playing time at first base because of Seattle's midseason acquisition of Mark Trumbo. The Mariners also have Jesus Montero at first base.
Miller, 26, opened the season as Seattle's starting shortstop, but he lost that role to rookie Ketel Marte and was transitioned to the outfield. He hit .258 with 11 home runs and 46 RBIs in 144 games while playing all three outfield spots as well as second, third and shortstop. Miller, a second-round Draft pick in 2011, hit .248 over 343 games with the Mariners over the past three seasons.
Farquhar, 28, was a big part of the Mariners' bullpen success in 2014, but he went 1-8 with a 5.12 ERA in 43 games over five stints with Seattle last year. In three seasons with the club, the Pembroke Pines, Fla., native was 4-12 with a 3.85 ERA and 18 saves in 155 games.
Dipoto noted the Mariners' surplus of first basemen made Morrison available, and Miller's situation was clouded a bit by Marte's arrival.
"Brad has been an attractive player for the other 29 teams," he said. "He's a trade target that others have come after. With the emergence of Ketel and how well he played, it gave us the ability to put Brad in play to answer other needs. He could have surfaced for us as a super-utility or outfielder, but we'd have been asking him to do something that was a new skillset. He'll get a chance to return to shortstop with Tampa [Bay]. This is a good baseball trade. Everybody walks away happy, including Brad Miller."
Morrison earned $2.7 million last year and figures to make about $4 million in his final season of arbitration eligibility before becoming a free agent in 2017. The other five players involved in the deal are all in their pre-arbitration years.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.