Castro, 20, entered the final day of the month batting .222 in June. Dave Keller, the Cubs' roving Minor League hitting instructor, said Castro's struggles are just part of growing up in the big leagues.
"I think people who get to see him every day at this level understand that it doesn't matter who you are, whether you're the guy in Atlanta [Jason Heyward] or Ike Davis or [Stephen] Strasburg or Starlin Castro, they all came with a lot of publicity," Keller said Wednesday. "There's a lot of positive reinforcement and a lot of hype for what type of player they have a chance to be at this level.
"They'll all go through their ups and downs, they'll all go through their peaks and valleys, because there's a big learning curve that happens here," he said.
What Keller explains to prospects is that advance scouts can figure out players' weaknesses fairly quickly.
"Those guys know what they're looking for, they know what they're talking about," Keller said of the scouts. "You don't get that in the Minor Leagues."
And they don't just watch the players at the plate, but also how they run the bases, how they play defense.
"After awhile, when they get that information, it's a matter of the player learning how to adjust, too," he said.
One difference this year is that Southern League teams now play five-game series. That gives opposing teams time to figure out a hot hitter -- and the hitter needs to learn to make the adjustment.
"If you don't have the ability, you won't make the adjustment at this level," Keller said. "If you do, you'll figure it out. [Castro] is a very energetic competitor who wants to do well, and that's something that's on his side and will help him get through some of the valleys he'll go through."