Tyler Wade, ranked by MLBPipeline.com as the Yankees' No. 10 prospect, was called up from Triple-A Scranton/Wilkes-Barre to take Castro's spot on the 25-man roster. New York also outrighted first baseman Ji-Man Choi to Triple-A Scranton/Wilkes-Barre.
Castro tweaked the hamstring on Wednesday against the Twins while fielding a slow roller at second base. Girardi said he didn't know the severity, but there was bruising on the back of his leg after Friday's game, which indicated he needed more time to rest and recover.
"He argued he was fine and he could play, and he's still arguing he's fine and he can play," Girardi said. "But our concern is, if he really tears it, then you have a real problem. So we're hoping it won't be long and we'll get him back. His strength is still there, but there's a problem."
Girardi said he didn't think Castro returned too soon from his initial injury, a Grade 1 strain that sidelined him from June 26 until he returned last Sunday.
"He was doing that really to protect himself," Girardi said of Castro not appearing capable of running at 100 percent. "Knowing that … when I don't have to run hard, there's no reason to run hard. I think we gave him substantial time."
Girardi said the initial plan is to platoon Ronald Torreyes and Wade at second base. The right-handed-hitting Torreyes is in line for more work against lefties and the left-handed-hitting Wade will match up more against righties.
Wade was called up during Castro's previous DL stint and hit .107/.219/179 in 32 plate appearances before being sent back down. He hit .304 with a double, a triple, a home run and three walks since returning to action with Scranton/Wilkes-Barre on Sunday.
"I thought, though, he did his job," Girardi said of Wade's performance during his initial callup. "What I saw, the more at-bats he got, the better they got and things started to slow down. We talked about that before, it's normal your first time up. Everyone goes through it. The next time up, you hope they're more prepared."
Girardi said the club hopes Castro undergoes a speedy recovery and the Yankees don't need to rely on their rookie infielder too heavily. But with Castro's injury history, they will be cautious.
"He's had issues and he's had issues before we got him," Girardi said. "And we're doing everything we can to protect him and stay away from it. But sometimes players that have hamstring pulls its becomes compromised and the chances of it happening again are greatly increased."
Josh Horton is a reporter for MLB.com based in Seattle. He covered the Yankees on Saturday.