Castillo signed a minor-league contract with the Marlins and was added to the organization's 60-man roster pool Aug. 3.
The 36-year-old will bolster Miami's depth at catcher, but he'll remain at the organization's alternate training site for the duration of 2020 unless the Marlins require another backstop at some point. The Marlins are already without two of their top three catchers in Jorge Alfaro (COVID-19) and Chad Wallach (undisclosed).