Castillo, nursing a sprained left ankle, might play Saturday night against the San Diego Padres, but manager Jerry Manuel said, "I'm more looking at [Sunday]."
Sheffield will take batting practice to determine if he will be available to pinch hit. Because Sheffield is still recovering from a hamstring injury he aggravated Wednesday, if he reaches base the Mets would insert a pinch runner, Manuel said.
Castillo sprained his left ankle when he slipped and fell in the dugout Tuesday night against St. Louis. He was on his way back to the bench when he tumbled down the dugout steps and needed to be helped up the tunnel to the clubhouse by some teammates.
Anderson hit .251 with one home run and 23 RBIs in 77 games with Washington. He spent parts of four seasons in the Mets organization, including major league stints in 2006-2008.
To make room for Hernandez, the Mets designated infielder Angel Berroa for assignment. Right-handed pitcher J.J. Putz was transferred to the 60-day disabled list to make room on New York's 40-man roster.
Right-hander pitcher Mike Pelfrey's wife, Angela, had a baby boy named Chase on Friday. Pelfrey, originally scheduled to start Saturday night against San Diego, will rejoin the team in Arizona and start Monday night in the opener of a three-game series.