"I had several options but chose the best, from the point of view that I will have a chance to play," Castillo told ESPNdeportes.com's Enrique Rojas from Miami. "I am excited to join a great team that could win it all this year. I think I can help them and fight to prove it on the ground."
"I have nothing against the Mets, but now I'm on the other side," he added.
The signing didn't shock Mets general manager Sandy Alderson. "It wasn't a surprise. It wasn't unanticipated," he told reporters.
Castillo won't make his debut for the Phillies on Tuesday because he has personal matters to attend to before reporting to his new team, a source told ESPN The Magazine's Buster Olney.
Concerns over Chase Utley's knee have the Phillies making contingency plans. Utley told reporters Sunday that he is "moving in the right direction," but he is battling patellar tendinitis, chondromalacia and bone inflammation and could open the season on the disabled list. He has yet to appear in a spring training game.
Castillo has battled his own knee problems and batted only .235 last season, his fourth with the Mets. He became a target of Mets fans' ire, especially after his dropped pop fly with two outs in the ninth inning cost the Mets a win against the city rival Yankees in a 2009 interleague game.
Phillies catcher Brian Schneider, who played with Castillo in New York, said he believed a change of scenery would help his new teammate.
"I think a change will be good for him, I respect him as a player," Schneider said. "He's been in the game a long time and he's done a lot of great things. I think this is a good opportunity for him with what's going on here."
While the change could help Castillo, he'll have competition for playing time in Philadelphia's infield. The Phillies already have Wilson Valdez, Josh Barfield, Pete Orr, Michael Martinez and Delwyn Young battling for time at second base in Utley's absence.
Valdez is batting .439 (18-for-41) in spring training with three doubles and one home run. Castillo was hitting .286 in 28 at-bats with the Mets before he was let go.
The Mets ate the final $6 million on Castillo's contract to cut him loose. With the minor league deal, the Phillies can give him a look without making a major commitment.
ESPNdeportes.com's Enrique Rojas and The Associated Press contributed to this report.