After spending the previous four years with the Pirates, Castillo signed as a free agent for $650,000 with the Marlins on Dec. 24, 2007.
The Venezuelan native was brought in to compete for the starting third base job vacated after Miguel Cabrera was traded to the Tigers. He has been battling with Jorge Cantu, who entered Friday leading the National League in hits (22) in Spring Training.
Dallas McPherson was initially in the hunt for the starting job, but he's been hampered by injuries and is expected to open the year at Triple-A Albuquerque.
Castillo got off to a slow start at the plate, but has since picked things up and was batting .283 entering Friday, when he was in the starting lineup against the Astros.
"I'm a little surprised; I know before I wasn't hitting good," Castillo said on Friday morning. "I'm surprised that I was put on waivers. I don't know what my situation is right now. I'm working hard. I want to stay here."
If for some reason Castillo isn't with the Marlins or another MLB club when the season begins, he may have an option to play in Japan.
"Before I signed with the Marlins, two teams from Japan, I don't know their names, they called me and asked if I would play for them," Castillo said. "I'm 27 now. They have good players there, but I'm waiting for [the Marlins'] decision."
The Marlins are not commenting on Castillo's waiver status, or even confirming he is on waivers.
Manager Fredi Gonzalez spoke generally about Castillo's performance.
"He's done a terrific job; he's a terrific defender," Gonzalez said. "He's made some [nice] plays at third base. He's swinging the bat OK."
The Marlins are looking to improve defensively, and Castillo has been a standout in the field at third.
"I'm happy here," he said. "I'm working hard on my defense. Before, I wasn't hitting. I'm waiting for the decision, whatever they say."