By Bruce Levine–

CHICAGO (CBS) — The White Sox are engaged in discussions with the San Diego Padres for veteran pitcher James Shields, according to baseball sources.

Entering Sunday, the White Sox were in the midst of a slump that caused them to fall out of first place for the first time since mid-April. They’re in dire need of another rotation pitcher to stay in the hunt of a hotly contested AL Central race.

The main element of a trade getting completed is the $58 million left on Shields’ deal, sources said. His contract runs through 2018 with a club option for 2019.

The White Sox won’t have to give up any talent on their big league roster, a source said. Right-hander Erik Johnson — who has made two starts for the White Sox this season and is now back in Triple-A — would most likely be in any deal between the two clubs. San Diego asked for Chicago’s top prospect, shortstop Tim Anderson, but was rejected. Anderson won’t be a part of a trade if one is agreed upon between the two clubs.

The Padres would be a nice place for the 26-year-old Johnson, with a cavern-like ballpark and some larger stadiums in the NL West perhaps allowing the flyball pitcher to flourish more. A second-round pick in the 2011 draft, Johnson is 2-1 with a 3.21 ERA at Triple-A Charlotte. In his two major league starts, he was 0-2 with a 6.94 ERA in 11 2/3 innings.

Other lower-level White Sox prospects would be involved if a trade is made. Chicago wouldn’t be assuming all of the money. White Sox management does have excess money with the sudden retirement of Adam LaRoche in spring training freeing up $13 million to use toward Shields contract. Projecting out further, John Danks’ $14.25 million comes off the books after 2016. He was released in early May.

The durable Shields would appear to be a quality right-hand pitching complement to go with the White Sox’s three solid left-handers in the rotation. Mat Latos would drop to the fifth spot in the rotation. Shields would likely be sandwiched between Chris Sale and Jose Quintana if acquired.

Shields has thrown 200 or more innings in eight straight seasons. He has a 2-6 record for the last-place Padres this season, but he his 3.06 ERA is a better indicator of the type of pitcher the White Sox would be getting.

The 34-year-old Shields is 129-103 lifetime with a 3.72 ERA.

Bruce Levine covers the Cubs and White Sox for 670 The Score and CBSChicago.com. Follow him on Twitter @MLBBruceLevine.