Castro signed a minor-league contract with the Royals on Saturday and received an invitation to spring training.
Castro signed a contract with the Toros de Tijuana of the Mexican Baseball League on Wednesday.
The Rockies sent Castro outright to Triple-A Albuquerque on Friday.
Castro went 2-for-3 with two RBI and a stolen base in Sunday's 10-5 loss to the White Sox.
Castro is out of the lineup for Wednesday's game against the Padres.
Castro is absent from the lineup Friday versus the Athletics.
Castro is not in the lineup Tuesday at Washington.
Updating a previous report, Castro will start at second base and bat ninth in Monday's game against the Nationals, Thomas Harding of MLB.com reports.
Castro is out of the lineup for Tuesday's game against the Nationals.
Castro is out of the lineup for Sunday's game against the Marlins.