Bleacher Report's Scott Miller first reported the sides were nearing an agreement.
Galvis, 28, has never been renowned as a particularly potent player at the plate, and the numbers bear that out.
Dating back to his MLB debut in 2012, Galvis has slashed a cumulative .245/.287/.372. That said, he has shown increased pop each of the past two years, with an OPS of .690 in 2017.
After mustering 20 home runs during his first four seasons, Galvis has gone deep 32 times over the past two years. He's also piled up 128 RBI during that stretch compared to a mark of 105 between 2012-2015.
More than anything, Galvis has solidified himself as a one of MLB's premier defensive shortstops over the past few years.
Dating back to the start of the 2015 season, Galvis ranks sixth among all players at the position with a 19.3 UZR, according to FanGraphs. He's also posted fielding percentages superior to the league average each of the past two seasons.
In 2017, Galvis registered a mark of .989 over 1,359.1 innings at shortstop—a number that comfortably trumped the league average of .973.
For the Phillies, parting ways with Galvis will open the door for fourth-ranked prospect J.P. Crawford to take over as the starting shortstop.
Crawford, who made his MLB debut on Sept. 5, slashed .214/.356/.300 with four doubles and six RBI over his first 87 plate appearances.
De Los Santos, meanwhile, provides the Phillies with another quality pitching prospect who could develop into a mid-rotation starter with some more refinement.
In 24 starts with Double-A San Antonio last season, the 21-year-old went 10-6 with a 3.78 ERA, 1.193 WHIP, 139 strikeouts and 48 walks.