SEATTLE -- Jerry Dipoto's plan to rebuild his roster with an eye toward the future got serious on Monday as the Mariners' general manager uprooted the man known as "Big Maple," dealing standout left-hander James Paxton to the Yankees for a trio of prospects -- left-handed pitcher Justus Sheffield, right-hander Erik Swanson and outfielder Dom Thompson-Williams.
Dipoto said about two-thirds of the Major League teams checked in on acquiring Paxton over the past few weeks, and the Mariners parlayed that into a pair of young pitchers they believe could be part of the rotation for years, as well as a promising young position player.
"Pax did draw a ton of interest," Dipoto said. "This is something we worked on since early October, when we determined the idea of where we wanted to go with our roster -- the idea of taking a step back to take two steps forward."
Dipoto said the Mariners are targeting a push toward the 2020 and '21 seasons as they accumulate promising young players with longer windows of opportunity. Paxton had two years until he hits free agency, while the three newcomers all have six years of team control.
Sheffield, the No. 31 overall prospect per MLB Pipeline who was also No. 1 in the Yankees' system, is a 22-year-old who is close to being Major League ready after going 6-4 with a 2.56 ERA in 20 games (15 starts) at Triple-A last season. He appeared in three games with the Yankees in relief in September.
"Justus Sheffield has an unquestionable prospect pedigree," Dipoto said. "With a combination of high-end velocity to go along with an advanced slider and a developing changeup, we think Justus has a chance to pitch at the upper portion of our rotation soon."

Swanson was the Yankees' No. 22 prospect as a 25-year-old, who also appears close to being ready for a Major League opportunity. Thompson-Williams is a 23-year-old who batted .299/.363/.546 with 22 homers and 74 RBIs in 100 games between Class A Advanced and low Class A last year.
Dipoto said he expects both young pitchers to compete for rotation spots at some point next season, while Thompson-Williams caught the Mariners' eyes as a change in his swing approach resulted in some dramatic improvement last season.
"He's a five-tool athlete who has just broken out and started to scratch the potential of what he might be," Dipoto said. "Like Jake Fraley or Mitch Haniger, he fits the criteria of a guy who has experienced some swing change, and it's resulted in some significant impact. He was hitting the ball a lot harder and more consistently. We're excited about his future as well."
The Astros' refusal to include Forrest Whitley in their offer for Paxton precipitated the Mariners' decision to trade Paxton to the Yankees, a source told MLB.com's Jon Paul Morosi.
Paxton figured to be Dipoto's prime trade chip this offseason. The 29-year-old lefty, who is eligible for arbitration for two more seasons, is projected by MLB Trade Rumors to earn $9 million in 2019.
The lanky Canadian went 11-6 with a 3.76 ERA in 28 starts for the Mariners last season, with 208 strikeouts in 160 1/3 innings. He threw the sixth no-hitter in club history on May 8 against the Blue Jays at Rogers Centre, a week after racking up a career-high 16 strikeouts against the A's.
"It seems to me that Seattle is trying to go young right now and building for the future," Paxton said in a conference call with Yankees reporters. "My window is only for the next two years through arbitration, so I think that they're going young. That being said, my experience in Seattle was fantastic. It was really special. Seattle will always be a special place in my heart. I'll always have a love for the city and wish them nothing but the best."
Paxton's biggest issue throughout his career has been staying healthy, but he provides the Yankees' rotation a huge boost as potentially one of the premier left-handers in the American League.
Paxton went 41-26 with a 3.42 ERA, 617 strikeouts and 168 walks in 102 starts over six seasons with Seattle, but he has made seven trips to the disabled list in the past four seasons.
Dipoto was willing to move Paxton as he attempts to shift the Mariners' focus to younger players with a longer window of opportunity in order to build around Haniger, Marco Gonzales and Edwin Diaz. Last week, the Mariners traded catcher Mike Zunino, who like Paxton has two years of team control remaining, for young Rays center fielder Mallex Smith in a similar move.
Paxton is a blockbuster deal, however, and he brings a big return in a pair of young starting pitchers.
Sheffield was a first-round pick by the Indians (No. 31 overall) out of high school in the 2014 Draft, and he was shipped to the Yankees in '16 as part of a four-prospect package for Andrew Miller.
Sheffield said he's never been to Seattle, but joked about the rain and "will go shopping tomorrow" for a rain coat. As for what Mariners fans should expect from him?
"Just kind of a bulldog mentality," Sheffield said. "No matter what jersey I'm wearing, I'll go out and compete and do what I can do for my team that day. My ultimate goal is to pitch deep in games, have a lot of innings under my belt and just come and compete."
Swanson was selected by the Rangers in the eighth round of the 2014 Draft out of Iowa Western Community College, then he was traded to the Yankees in '16 in a three-prospect deal for Carlos Beltran.
Swanson and Thompson-Williams were teammates for a year at Iowa Western, and he's also become friends with Sheffield after pitching alongside him in the Minors this past season.
"We just pushed each other and picked each other's brains in the dugout all the time," Swanson said. "Justus is a phenomenal person and really good teammate. I'm pretty excited that he's coming over with me."