On the day he officially was eligible to sign, Castillo quickly narrowed his focus and is poised to join the Philadelphia Phillies, according to a baseball source.
According to CSNPhilly.com, Philadelphia's offer is a minor league contract.
The Phillies' second base situation is unsettled because of All-Star Chase Utley's knee woes. The All-Star has not played in any of the Phillies' exhibition games this spring because of patella tendinitis in his right knee.
Castillo, 35, hit .235 with no homers and 17 RBIs last season in 247 at-bats.
The Mets began the process of releasing Castillo on Friday, despite owing him $6 million in the final season of a four-year, $25 million deal. Castillo's new employer will only be responsible for the major league minimum.
Adam Rubin covers the Mets for ESPNNewYork.com.