Castro went on the disabled list last week with a tear of his right meniscus.
"It's disappointing, and hopefully we'll get a better idea of the length of rehab after the procedure," Molitor said.
Castro was hitting just .143 when he went on the DL.
Castro has missed time with injuries to his right knee in the past and missed the 2011 season with a torn ACL.
Molitor had some good news on another injured Twin, however.
Ace right-hander Ervin Santana, out all season after having debridement surgery on his right middle finger in February, threw to hitters Monday for the first time since the surgery.
"He had a good day," Molitor said.
The Twins are hopeful that Santana will have an extending spring training start next week.