This news isn't surprising, but it's interesting for a couple of reasons.
One, the Mets still owe Castillo $6 million.
And two, Castillo, for all his limitations, might still have been the best second baseman on the 40-man roster.
In Castillo's absence, the position will be filled by one or more personages from the following group: Daniel Murphy, Brad Emaus, Justin Turner, and Luis Hernandez, with the unheralded Hernandez supposedly in the lead because he's got the best glove.
In the strictest sense, this isn't really a "baseball move," but is rather designed to suggest that business isn't as usual. With a rebuilt front office and a new manager, statements must be made after consecutive losing seasons.
The Mets might win more games than they lose this season, but given their pitching rotation that doesn't seem likely. Win or lose, they're not going to the playoffs and Luis Castillo certainly wasn't going to be around when they do return to the postseason some day.
Out with the old, in with the new, and maybe seeing Castillo cleaning out his locker will send a message to someone who will be around when this team is ready to win again. And fairly or not, Castillo's legacy after three-plus seasons with the Mets will always be that game-losing error.