The 35-year-old Castillo signed a minor league deal with the Phillies on Monday and could earn regular playing time with Chase Utley out because of a right knee injury.
"I'm trying to find a job here," Castillo said. "I want to compete, I want to win the job. I want to play. I have 10 days to prove it and get ready."
Castillo was let go by the Mets despite being owed $6 million in the final year of a four-year deal. If he makes the team, the Phillies would only be responsible for $414,000, the major league minimum.
The three-time All-Star hit .235 last year and sat much of the second half after returning from a heel injury, falling out of favor with the Mets' staff. He was in a competition with four others for the second base job this spring with New York.
Castillo hit .235 with Mets in 2010 but was limited to just 86 games while battling a right heel injury.
He met with first-year Mets manager Terry Collins when spring training began to talk about his future with the team.
"I needed a chance to play, he wouldn't give me the chance to play," Castillo said. "It didn't work out. I told him if he didn't give me the chance to play, I don't know if I have to be on the team. He decided to release me and wouldn't give me the opportunity, and I'm here."
Castillo joins a group of five others who have spent time at second base while the Phillies await Utley's return. Utley hasn't played in any of the Phillies' exhibition games this spring.
Utility reserve Wilson Valdez is close to a lock for a roster spot, but Rule 5 pick Michael Martinez and non-roster players Josh Barfield, Delwyn Young and Pete Orr will all compete for a job with Castillo.
Phillies general manager Ruben Amaro Jr. said bringing in Castillo amounts to a weeklong tryout before the season begins on April 1.
"He's not going to go to the minor leagues," Amaro said Monday. "He's either going to make our club or be released."