Cubs player development director Oneri Fleita called Castro with the news early Tuesday and someone showed him the photograph.
"[Fleita] told me this magazine is nice," Castro said Tuesday.
The youngest player in the Major Leagues at 21, Castro collected 40 hits in the first month, tying the Major League record for most April hits by a player 21 or younger. Alex Rodriguez had 40 hits in April 1997.
"He's fun to play next to," Cubs second baseman Darwin Barney said of Castro. "He's so athletic that you can do a lot of things. We both think we have range and we love playing together.
"Seeing him on the cover is pretty awesome. It's really cool and I'm happy for him. The guy's worked really hard and his growth is unbelievable. When I think about where I was at 21, I was still questioning myself and trying to learn what kind of player I was. It's probably not the last time he'll be on the cover of some magazine."
However, there is the matter of the Sports Illustrated jinx.
"How many do you have to deal with?" Cubs manager Mike Quade said. "You have the sophomore [jinx], the S.I. [jinx]. If there's two jinxes, do they cancel each other out?
"I know he's well aware of the notoriety of being on the cover of Sports Illustrated. He still understands that doesn't make him king of all shortstops. He still has work to do to get better."