Castro is only 5-foot-8 but can hit up to 95 mph with his fastball. He has four years of professional experience but has never pitched above Class A.
The Rangers acquired him in a trade with the Kansas City Royals after they took him with the first pick in the Rule 5 draft from the White Sox.
He'll be the long reliever but the Rangers are going to carry him just because they think he has a great future.
"He can fill a role but on a team expecting to contender, he has to be able to do his part," Daniels said.
The Rangers haven't had a Rule 5 draft pick break with the team since pitcher Ramon Manon in 1990. That was the year Spring Training was shortened by the owners' lockout and rosters were expanded to 27 players for April. Manon ended up going back to the Yankees at the end of April.
Alfonseca, Feldman round out bullpen:
Antonio Alfonseca has made the Rangers as one of their right-handed setup relievers. He joins right-handers Joaquin Benoit and Akinori Otsuka and left-hander Brian Shouse as the Rangers primary late-inning options in front of closer Francisco Cordero.
Alfonseca was 1-1 with a 4.94 ERA in 33 games with the Florida Marlins last year while missing three months with a stress fracture in his elbow.
Pitching coach Mark Connor pushed him in Spring Training to test his health and Alfonseca pitched in 11 games, the most on the Rangers. Opponents hit .256 off him.
"He held his stuff," manager Buck Showalter said. "His last two outings he had had his best fastball."
Right-hander Scott Feldman also made it as the Rangers designated left-hander Erasmo Ramirez for assignment.
Roster set: The Rangers rounded out their roster by acquiring the contracts of outfielder Adrian Brown and infielder D'Angelo Jimenez. Both had been in camp on Minor League contracts.
Outfielder Adam Hyzdu was reassigned to minor league camp. The Rangers kept Jimenez because it allows them to use infielder Mark DeRosa in the outfield if needed.
The Rangers also optioned pitcher John Rheinecker to Triple-A Oklahoma. He was acquired Friday from the Oakland Athletics.
John Wasdin has signed a contract with Triple-A Oklahoma and will pitch in their rotation. ... Jon Leicester is also going into the Oklahoma rotation. He was a reliever in camp but Showalter said, "If we stretch him out, we can bring him back either way." ... Josh Rupe has started throwing again in Spring Training for the first time since coming down with elbow inflammation. He is still a month away. ... Gary Matthews Jr. had a good day hitting back at the Spring Training complex and the Rangers expect him to be ready for a rehab assignment on Thursday at Oklahoma. ... C.J. Wilson threw 21 pitches in a Minor League exhibition game but the reports weren't good. Said Showalter, "His command was a little off. He was out of whack mechanically." ... Pitcher Omar Beltre, still stuck in the Dominican Republic because of a visa problem, was placed on the restricted list.