The key to a Red Sox-Padres Pablo Sandoval trade could be Melvin Upton

There is a slim chance of the Red Sox trading Pablo Sandoval to the Padres. San Diego wanted to sign him before the 2015 season, and they were scouting him and then discussing the possibility of a trade just last week. However, there are some real barriers that could keep a deal from happening, like Sandoval's salary and room on the roster for him in San Diego.

With that being said, there is one player who the Red Sox and Padres could swap along with Sandoval that would make a trade sensible for both sides. Melvin Upton Jr. isn't under contract for as long as Sandoval, but he's similarly expensive. If there's one area the Sox need depth in, it's the outfield, and Upton could provide that.

Upton wouldn't just make sense for the Red Sox -- which we'll get to -- but his removal from the roster would also make Sandoval possible in the Padres' lineup. As of right now, they have Yangervis Solarte at third base. He's a below-average defender in the infield who hit himself into a starting role anyway when no one else could. Upton is currently in left for the Padres, even though his glove could work elsewhere -- San Diego could swap Solarte to left, where the former utility player has only briefly played in the past, and put Sandoval at third should they exchange Upton for him.

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It wouldn't be a move meant to help the Padres win right now, necessarily -- not that the Padres are trying to win right now -- but Upton is a player under contract for just two more years with $31.9 million still owed him, whereas Sandoval has four left and $75 million to go. If the bet is on Sandoval rebounding with a fresh start in a less-demanding environment -- the kind he was in prior to signing with Boston -- then more Sandoval would be considered a positive here. Throw in that the Sox would likely cover a portion -- okay, a large portion -- of his salary during the years after Upton's own deal expires, and the Padres could very well be fascinated by this admitted risk as they head towards a rebuild.

The Padres plan to improve internally for the most part -- one of the worst-kept secrets going right now is the $30 million in commitments (and another $30 million in penalties) they have prepped for the 2016-2017 international free agent period. They already made a deal with the Red Sox for closer Craig Kimbrel that brought them a top-15 prospect and a top-50 one who could climb even higher. Players like Matt Kemp, James Shields, Andrew Cashner, and even Tyson Ross (if they don't plan on re-signing him after 2017) could be on the way out this July in order to bring in even more prospects and young pieces in to surround the likes of Wil Myers, Derek Norris, Austin Hedges, Hunter Renfroe, Colin Rea, Margot, Guerra, and more. Sandoval could fit in with this group as a veteran at a position where they don't have an answer, one who won't be old by the time the kids show up and start to play as hoped.

You might be wondering why the Red Sox would want Upton, though, and that's a fair question, especially since the chances anyone outside San Diego paid attention to the performance of a part-time Padres outfielder in 2015 are basically nil. Upton hit .259/.327/.429 for a 112 OPS+ in 2015, over 87 games and 228 plate appearances. He's not the budding star he used to be, but he's a fine defender still (especially now that he's in left instead of center) and showed that there's still some life left in his bat despite what his depressing time with the Braves suggested.

The Red Sox are currently considering sending Rusney Castillo down to Triple-A because they don't trust him to start in the majors right now. They might not ever trust him in anything besides a bench role, either, as President of Baseball Operations Dave Dombrowski thought he was nothing more than a fourth outfielder on a good team back when the Red Sox signed Castillo in 2014 -- Dombrowski was still with the Tigers then, and nothing Castillo has done since has disproved his initial take.

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Brock Holt has been the team's left fielder during the first two games of the season, and while their versatility isn't compromised by his playing there against right-handers, there is room to improve. Adding Upton to the mix would give them someone with more experience and success than Castillo, and since he would basically cost what Sandoval would over the next two seasons, it wouldn't feel like there was dead money on the bench like there is now. Upton, Jackie Bradley, and Mookie Betts is also a better defensive outfield, and maybe as good as what they'd have if Castillo were out there, and might be the best bet offensively, too. Holt still hasn't finished a year as an above-average hitter, even if he has stretches where he looks like a diminutive Ted Williams.

The Sox would get Brock Holt back into his most productive role as the backup infielder and fifth outfielder. They would have an actual outfielder starting in left field in Melvin Upton, one who should benefit from the move to Fenway Park after years spent in three different pitcher-friendly stadiums. They would still have the depth of Castillo in the organization, should Upton falter. And yes, they would still be paying a substantial amount of money for Sandoval, but the Sox might be able to confine that money to 2018-2019 for the most part, given they would be taking on Upton's similar-ish salary over the next two seasons.

It's a deal with risks and positives for both sides, and one you could see working out depending on if the two teams could come to terms on how much money the Red Sox need to send over and who the other player or players in the deal would need to be. It makes far more sense for both sides than swapping out James Shields or demanding the Red Sox send over top prospects along with Sandoval, as the Sox get help for now while the Padres bet on getting help for later -- remember, Sandoval is only 29 years old, so if he rebounds with a change of scenery he'll still only be 32 years old when his current contract ends. There's a reason Sandoval was offered $95 million by the Red Sox, and why the Padres reportedly offered him even more.

The Red Sox might not want to give up on Sandoval, at least not yet. If Travis Shaw continues to hit as the starting third baseman, though, and the outfield situation is clearly still in need of assistance, the Padres and Red Sox might want to talk about an Upton-for-Sandoval swap. It's about the only deal that would make sense between the two teams given their respective windows and future plans, and would be yet another reminder that no contract is immovable in this game.