The Hot Stove season has seen a flurry of action on both the free-agent market and the trade front. Some very impactful players have switched teams, filling some gaping holes while creating others.
But which players represent the biggest upgrades for their respective team entering the 2016 season? That is to say: Which players have the greatest difference in value over the player who manned that spot last season?
Steamer -- a player-projection formula used to predict future performance based on trends and historical comparisons -- projects the WAR for every player in the 2016 season. These are the 10 acquisitions who offer their team the biggest upgrade from last year to this year:
If Span is healthy, he's an obvious upgrade over Aoki in San Francisco, but after he played just 61 games last season because of a hip injury, that health is still a question mark. Still, Span more than fills the void in the Giants' outfield that was created when Aoki left for the Mariners. Span's speed and ability to reach base make him an ideal fit at the top of San Francisco's order, in front of Joe Panik and Buster Posey.
Zimmermann is a fantastic fit for Detroit for several reasons -- mainly the fact that he slots in at the front end of the rotation, while taking some of the pressure off the club's young arms at the back end. Plus, the Tigers didn't need to surrender a first-round pick to attain him. Simply put, Zimmermann helps complete Detroit's rotation -- a rotation that won't exactly miss Simon, who posted a 5.05 ERA and a 1.44 WHIP in 31 starts last season.
It can't have been easy for the Cubs to part with Castro, who was a critical part of their late-season surge in 2015. But there's no doubt Zobrist represents an upgrade -- specifically for the next season or two. Zobrist figures to be a perfect fit as a veteran presence on a team full of youngsters. And his ability to reach base (he's done so at a .355 clip throughout his career) will play nicely in a lineup with plenty of sluggers capable of driving him in.
The Angels invested in their future when they traded for Simmons in November. After all, Simmons is under team control through the 2020 season, while Aybar is set to become a free agent next offseason. Still, Simmons is an upgrade in Anaheim for '16. The offensive projections are basically a wash, but Simmons' elite defense is the biggest reason he should be worth around two wins more than Aybar in '16.
Johnny Cueto does not make this list, and perhaps that's unfair to Cueto. He figures to have a much bigger impact on the Giants in 2016. But for the purposes of this exercise, Cueto (projected 3.1 WAR via Steamer) replaces Mike Leake (2.2) -- the best pitcher who left San Francisco's rotation this offseason. While that's clearly a notable upgrade, it's not as impactful as Samardzija filling Vogelsong's void. Because of Leake's departure, the Giants' rotation didn't truly get a significant boost until Samardzija's signing complemented Cueto's.
The D-backs added Greinke and Shelby Miller to their rotation, and they get a healthy Patrick Corbin to start the year, so there's no question the starting five will be much better for Arizona. Greinke, however, seems slightly devalued here, because his FIP-based WAR is lower than his RA9-based WAR. He's one of the best in the game at run prevention and figures to play the central role in the D-backs' revival.
The Cubs had a weakness in the bottom half of their rotation, and by signing Lackey, they turned that weakness into a strength. He slots in nicely as the No. 3 starter, with Kyle Hendricks and Jason Hammel each moving down a slot -- where they probably belong. Plus, the signing allows Wood to move back to the bullpen. Lackey may not have the same clout as the rest of this list's top five, but he's clearly a significant upgrade within the Cubs' rotation.
Theoretically, Price represents an even bigger upgrade than 3 WAR. Think of it this way: Boston traded Wade Miley for Carson Smith -- a very good reliever. It's almost as though you can add an extra inning of work onto each of Price's starts. Of all the players to switch teams this offseason, Price's projected WAR is the highest. Given the wide-open nature of the American League East -- and the fact that Price's departure left Toronto without a bona fide ace -- his signing could pay major dividends for Boston in 2016.
Notice a common theme here? Heyward represents the fourth straight player to sign with a team in the same division. That gives each of these signings a slightly higher value, as the roster of a direct competitor is weakened. In this case, the Cubs poached Heyward from the Cardinals. And while Fowler was a serviceable center fielder for Chicago, there's no question Heyward -- one of the best all-around outfielders in the game at just 26 -- represents a massive upgrade for 2016 and beyond.
How badly did the White Sox need someone new to man the hot corner? Well, since 2011 -- Frazier's first season in the Majors -- Chicago third basemen have been worse than replacement level, posting a -0.5 WAR which is easily the lowest in the Majors. Frazier, meanwhile, owns a 15.5 career WAR, good enough for ninth in the big leagues among third basemen in that time. It's certainly debatable whether the White Sox busy offseason makes them contenders right away. But there's no denying the upgrade Frazier represents.
AJ Cassavell is a reporter for MLB.com. Follow him on Twitter @ajcassavell. This story was not subject to the approval of Major League Baseball or its clubs.