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The deal: The Yankees acquire LHP James Paxton from the Mariners for LHP Justus Sheffield, RHP Erik Swanson and OF Dom Thompson-Williams.

Why the Yankees did it: Looking to bolster a rotation that ranked fifth in the American League in ERA in 2018, the Yankees added one of the premier left-handed pitchers in the majors — and one of the few quality starters available on the trade market. Paxton had the second-highest fastball velocity among lefty starters in 2018, behind only that of Cy Young winner Blake Snell, while going 11-6 with a 3.76 ERA and 208 strikeouts in 160 1/3 innings. His strikeout rate of 32.2 percent ranked fifth among pitchers with at least 100 innings, and his 3.40 ERA the past two seasons ranks 22nd among pitchers with at least 250 innings.

When he's on, Paxton will just blow the fastball past hitters, jam right-handed batters with a cutter and pile up strikeouts with his curveball. He had a 16-strikeout game (in seven innings) and a no-hitter in back-to-back starts in early May. There's no doubting the quality of Paxton's stuff, but the knock against him is his durability. He has never qualified for an ERA title, and his 160 innings and 28 starts in 2018 were both career highs.

His injuries have mostly been of the nagging type; he had a short DL stint in August after getting hit on the forearm by a line drive and then missed a couple starts in September with a virus. His conditioning has been questioned, and his home run rate did spike from 0.6 per nine innings in 2017 to 1.3 in 2018, which largely explains the rise in ERA from 2.98 to 3.76. After a slow start in April (5.12 ERA), he had a 3.43 ERA the rest of the season while holding batters to a .219/.260/.375 line.

The Yankees won 100 games in 2018, but all that produced was a wild-card berth, as they finished eight games behind the Red Sox. They haven't won the AL East since 2012, and the other day, general manager Brian Cashman said, "I don't think it really matters what we wind up doing, as long as we do well enough that we become the best team in baseball."

That has to be the goal. Sheffield is widely regarded as the team's top pitching prospect and currently ranks No. 31 on MLB.com's top-100 prospects list, but the Yankees need a finished product, and Sheffield isn't a finished product just yet. Paxton is under team control for two years and projects to an $8.5 million salary in arbitration. If the Yankees can trade Sonny Gray, that's basically a wash with Gray's projected salary … meaning there is plenty of money in the vault for another addition to the rotation in free agency, such as Patrick Corbin, Dallas Keuchel, Nathan Eovaldi or J.A. Happ.

Why the Mariners did it: They're caught in no-man's-land — winning 89 games in 2018 but being outscored and finishing 14 games behind the Astros. Oh, and they're the owners of a bloated payroll, thanks to the Robinson Cano, Felix Hernandez and Kyle Seager contracts (those three will make more than $71 million in 2019). More problematic, the Mariners have what is universally regarded as the worst farm system in the majors, with no immediate impact talent.

With that in mind, Trader Jerry Dipoto is trying to add some younger and cheaper talent without completely tearing down the roster. Sheffield gives Seattle a major-league-ready arm, and Swanson has also reached Triple-A, giving the team two candidates for the 2019 rotation.

Sheffield is a compact southpaw, kind of built along the lines of Jose Quintana of the Cubs. Listed at 6-foot, he's probably an inch or two shorter, but he owns a 92-97 mph fastball, slider and changeup. He had a 2.48 ERA in the minors last season, making five starts at Double-A and 15 at Triple-A, with 123 strikeouts and just four home runs allowed in 116 innings. Command is still an issue, as he walked 50 batters.

I spent some time with Sheffield in spring training, and he's what I would describe as a baseball rat. He's from Tennessee, and David Price was his favorite player growing up. The Yankees have praised his competitiveness. Batters hit .199 against him, with lefties hitting just .156. There's obvious top-of-the-rotation potential here if everything comes together. He'll certainly start the season in Triple-A to preserve some service time, which means he'll be under Mariners control for six-plus seasons.

You do feel a little for the kid. I asked him if he'd rather be with the Marlins organization — where he very well might have pitched in the rotation in 2018 — or the Yankees, and he said the Yankees, that his dream was to pitch at Yankee Stadium in pinstripes. At least he got to do that: He pitched one inning at home during a September call-up.

Swanson was an eighth-round pick by the Rangers out of an Iowa community college in 2014, and he went to the Yankees in the Carlos Beltran trade in 2016. His performance took a big step forward in 2018. He dominated Double-A, allowing just three runs in 42 2/3 innings, and he posted a 3.86 ERA in Triple-A. Overall, he fanned 139 with 29 walks in 121 1/3 innings, though all 10 home runs he allowed came in Triple-A. He's more of a performance than raw-stuff guy, with a 92-94 fastball, changeup and improved curveball. He's a big dude at 6-foot-3, 245 pounds, and the numbers suggest a guy who could compete for the back end of the rotation.

Thompson-Williams is a left-handed-hitting outfielder who hit .299/.363/.546 at Class A with 22 home runs, but at 23, he was a little old for the league and had a poor strikeout-to-walk ratio (102 K's, 33 walks).

Yankees grade: B+. They gave up only one premium prospect, which is a little surprising given that Paxton would have been in high demand for multiple teams. Big Maple isn't without risk, but adding a power lefty to Yankee Stadium is exactly what the rotation needed.

Mariners grade: B. Seven years of Sheffield for two years of Paxton has the chance to be a big payoff — if Sheffield pans out. I thought the Mariners would get a second prospect better than Swanson, but the numbers suggest that he could surprise. Still, it's a little bit of spinning wheels in the mud for a team that has been stuck there for years.