Negron was recalled by the Mariners from Triple-A earlier this month and has batted .217 in 10 games. Before joining Seattle, he was batting .310 with 12 homers and 61 RBIs at Tacoma.
Castro, an infielder, was signed by the Dodgers as a free agent last November. He was batting .241 with 11 RBIs in 45 games with Triple-A Oklahoma City this season.
The Dodgers need depth in the infield, as second baseman Kike Hernandez (left wrist) will likely be placed on the injured list Monday, according to manager Dave Roberts.
The Associated Press contributed to this report.