Castro tore his meniscus, was operated on Tuesday by Dr. Richard Steadman and had been expected to be sidelined for four-to-six weeks.
Minnesota said Wednesday that Steadman found damage to be more extensive than an MRI indicated. As a result, Steadman performed a full repair instead of a trim of the meniscus. Castro's meniscus was trimmed during an operation in 2011.
"There's only so much meniscus you can clean up as you go," Twins chief baseball officer Derek Falvey said Wednesday. "Certainly, MRI read for us that we thought it would be more of a clean-up, but you don't know until you go in there."
Falvey said Castro's rehabilitation will take five-to-six months, putting the 30-year-old on track to be ready for spring training. Castro had surgery to repair the anterior cruciate ligament and meniscus in 2011 and had a cyst removed from the knee in 2013.
"They feel really good about where he's at for the long term for his career and will allow him after this rehab to be in a good spot," Falvey said.
Castro has been on the disabled list since May 5. He hit .143 with three RBIs in 19 games and tried to play through knee pain for several weeks.
Minnesota is left with second-year player Mitch Garver and journeyman Bobby Wilson, who homered for the first time since 2016 in Tuesday's win, at the position.
Garver has hit .254 with two home runs in 20 games as Castro's backup this season. Wilson is back in the major leagues for the first time since playing for three teams in 2016.
"I have a lot of confidence," manager Paul Molitor said of Garver and Wilson. "Since Jason has gone down, they've both done some things to show they're able to help us win."
Falvey said he will pursue possible upgrades.
"More of the pursuit of what might be available across the game currently in organizations, I would say, rather than a free-agent conversation," Falvey said. "But we have been prepping for the depth piece, and we are now maybe expanding that search, knowing we're going to have the chance to have somebody impact us for the full season."
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