Twins manager Paul Molitor announced Wednesday that Castro had surgery to repair a torn meniscus and is "looking at a long rehab."
The surgery, performed Tuesday, was expected to involve removing a portion of Castro's meniscus. But Molitor told reporters Wednesday that the tear "was a little bit more significant."
"They decided to try to keep what he had left [of the meniscus]," Molitor told reporters, according to the St. Paul Pioneer Press. "They thought that was the best course of action for him as far as prolonging his career and productivity."
Molitor told reporters Monday that the Twins expected Castro to miss four to six weeks. But two days later, Molitor explained that Castro agreed to the season-ending surgery in order to prolong his career.
"Jason was made aware the day before that when you get in there, this is always a possibility," Molitor said. "He was on board if they had to do what they had to do that he wanted to protect his future."
Twins chief baseball officer Derek 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 ACL 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 was batting just .143 with one home run in 19 games this season, his second with the Twins. He has one year remaining on a three-year, $24.5 million contract with Minnesota.
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.
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.''
The Associated Press contributed to this report.