Castro didn't want to leave Tuesday's game after he was injured and tried to talk manager Rick Renteria into letting him take the field.
"He was in that hallway [near the Cubs' dugout], pretty upset when I told him to go in and see [the athletic trainer] and get evaluated," Renteria said.
Castro has played in 134 of the Cubs' 140 games, and only missed extended time when he went on the bereavement list. His goal this spring was to play 162 games.
"This is going to eat at him to be out," Hoyer said.
If Castro's season is done, he rebounded from a disappointing 2013, when he batted .245 with 10 home runs and 44 RBIs. This season, Castro hit .292 with 14 home runs, matching his career high, and 65 RBIs, and was named to the National League All-Star team for the third time.
Cubs strength coach Tim Buss spent a few weeks in the Dominican Republic with Castro this past offseason to prepare for the current season, and may do so again, Hoyer said.
"He's probably done better than most expected or anticipated, and maybe he's done as well as he's wanted to do," Renteria said. "In my mind's eye, I think he's had a very productive season."
Renteria talked to Castro Wednesday.
"His spirits are up and he said, 'I'll be back,'" Renteria said.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.