The jewel of the Padres' prospect haul is Naylor, a 19-year-old first baseman and the Marlins' first-round pick in 2015. Naylor was batting .269/.317/.430 for Class A Greensboro of the South Atlantic League.
The Padres also netted right-handed flame-thrower Castillo, who boasts a fastball in the upper-90s and a solid slider to complement it. The pitch mix suggests he'd make a quality big league reliever one day. But the 23-year-old has been a starter throughout his Minor League career, and he owns a 2.25 ERA in 100 innings this season for Class A Advanced Jupiter. The Padres plan to proceed with Castillo in a starting role.
Padres general manager A.J. Preller said that approximately 10 teams had called on Cashner, with half of them progressing into serious talks. The Padres, meanwhile, had been eyeing Naylor and Castillo and the rest of the deal simply fell into place after that.
"The big thing for us, on the prospect side, it was pretty clear for us that Naylor and Castillo were two names that we wanted to include," Preller said. "That's where the deal kind of developed into the bigger deal - to see how we could end up getting both those guys."
It appears likely Naylor will begin his Padres career at Class A Advanced Lake Elsinore, while Castillo will begin at Double-A San Antonio. That would be a jump for both of them, and it would make Naylor one of the younger players in the entire Cal League. Cashner, meanwhile, had long been rumored as a Padres trade candidate, and his past three outings sent his value skyrocketing. Despite his 4.76 ERA, Cashner has reverted to his old self during the second half, allowing five runs while striking out 23 over three starts.
"His last three outings, he threw the ball really well," Preller said. "It's definitely what he's capable of. … Teams were more interested, after the way he threw the ball the last three outings."
The hard-throwing right-hander was the longest-tenured Padre, but his contract was set to expire after the season. With San Diego out of contention, a deal seemed imminent.
The addition of Rea came as something of a surprise, as the 26-year-old rookie still has five years of team control. But it's likely his inclusion played a major role in the Padres' prospect haul. Rea has shown flashes of brilliance this year, but he owns a 4.98 ERA and a 1.46 WHIP.
"I don't think we wanted at all to part with Colin Rea. But when the opportunity to acquire those four guys came our way, we thought it was best as an organization to make the trade," said Padres manager Andy said.
With Rea and Cashner absent from the rotation, the Padres will likely hand the ball to long reliever Paul Clemens to fill one of those spots. They view Clemens, claimed off waivers from Miami in June, as a project with the makings of an effective starter one day.
Cosart, who owns a 5.95 ERA in four starts, will likely fill the other vacancy in the rotation. With the trade, San Diego has now dealt four of the five pitchers from its Opening Day rotation, with Tyson Ross (currently sidelined by shoulder inflammation) the only starter remaining.
The addition of Capps is undoubtedly a risk for the Padres, but it's one they can afford to take. Capps was one of the best relievers in the Majors last season, but he was shut down in August because of an elbow injury and underwent surgery in March. Capps -- who is best known for his unorthodox delivery in which he drags his back foot from the rubber and seems to pause before throwing -- allowed 18 hits and just four earned runs in 31 innings last season. He struck out 58 -- nearly two batters per inning -- and if he returns to that form, he could become San Diego's closer of the future.
"When he was healthy last year, we felt he was one of the top four or five relievers in baseball, honestly," Preller said. "He was dominant with big velocity and a breaking pitch."
As for Guerrero, he made his big league debut for the Padres on May 17, but he has mostly struggled between Double-A San Antonio and Triple-A El Paso. The fastball-heavy right-hander owns a 5.30 ERA in 32 Minor League relief appearances this year. He typically sits above 95 mph with his heater, but he has had command issues. Guerrero was ranked as San Diego's No. 13 prospect when the season began, but he dropped out of the Top 30 entirely with MLBPipeline.com's midseason update on Thursday.
Of course, that's largely because of the recent upgrades the Padres have made to their system. Seven of San Diego's top eight prospects have been acquired since the end of last season, and 18 of the top 30 overall. And that doesn't yet include Naylor and Castillo. Including Naylor (No. 4) and Castillo (No. 15), eight of San Diego's top nine prospects have been acquired since the end of last season, and 19 of the top 30 overall.
"We've talked a lot about building depth throughout the organization and just really having answers and ultimately having waves of players we feel can come up and produce for us at the major league level. I think we're starting to get to that point through the draft, through the international period, now through trades," Preller said.
AJ Cassavell covers the Padres for MLB.com. Follow him on Twitter @ajcassavell. This story was not subject to the approval of Major League Baseball or its clubs.