Jon Heyman of FanRag Sports first reported the acquisition. In return, the Padres will receive shortstop Fernando Tatis Jr. and right-hander Erik Johnson, according to Dennis Lin of the San Diego Union-Tribune.
The 34-year-old Shields is 2-7 with a 4.28 ERA in 67 1/3 innings pitched this season. Over his 11 starts, Shields has struck out 57 hitters, has walked 27, and has allowed nine home runs.
In addition, Shields has a 4.42 FIP and a 4.14 xFIP, according to FanGraphs.com, with the latter suggesting that he has been a better pitcher than his ERA advertises. Shields has been worth 0.4 fWAR.
The Padres signed Shields to a four-year, $73 million deal last offseason, in hopes that they would begin to contend.
Shields, who is owed about another $12 million this season, can opt-out at the end of the year. He still has $44 million guaranteed through 2018, with a $16 million option for 2019.
The Padres will pay more than $30 million of Shields' salary in the trade, according to Ken Rosenthal of FOX Sports. If Shields opts out of his contract after the season, the Padres' commitment to the White Sox will decrease appropriately.
The White Sox, who are 29-26 through their first 55 games this season, have been in the market for a mid-rotation starter during the month of May and into early June.
At the top of their rotation, the White Sox have the likes of Chris Sale (2.29 ERA) and Jose Quintana (2.13 ERA). Carlos Rodon (4.41 ERA), Mat Latos (4.02 ERA), and Miguel Gonzalez (4.09 ERA) follow them.
As a whole, the White Sox' rotation has been very good this season, having been worth 5.7 fWAR, good for seventh in the Major Leagues. Despite this, Shields could still be a worthwhile addition to add some much-needed depth in the middle.
In Tatis Jr. and Johnson, the Padres will help in their retooling efforts.
Johnson, 26, has a career 4.50 ERA in 18 Major League starts over four seasons. Tatis Jr., 17 years old and the son of former Major Leaguer Fernando Tatis, signed with the White Sox during the last international signing period.