Daylight!

For a lot of players that are good and the team knows it, they are focusing on the things they don't do well.   That's why a pitcher with so-so stats may be the best pitcher on the team.  They know he's got a plus fastball and could strike out every body in the minors with it, but they make him throw the change every other pitch even if it sucks until he figures it out, because a fastball alone isn't good enuf in the show.    

Lets say for instance, Young Napoli can't hit off speed stuff, so they tell him to lay off fastballs no matter what and to swing only at the off speed pitches.  

Sometimes, it's also a mechanics or mental breakthrough that happens and suddeny the guy becomes instantly better.  They might fix a hitch in his swing that he has always had and suddenly he can rake, or he suddenly learns how to pick up the spin on the breaking ball before the ball starts to break.  Tiny little things can make massive differences in baseball.  

Conversely, there are sometimes very athletic players who on talent alone can mash minor league pitchers because they throw so many mistake pitches, but in the show, it's much nastier stuff and far fewer fat pitches.  Same with pitchers, a 95 fastball that  often sails over the plate and a mediocre curve can dominate in the minors, but it'll get you killed in the show.

but prospect shmospect, the show box score #s don't lie, he's getting rocked.  He might me good someday but not yet. The talent differential between the minors and the show is so great, you never really know what you're going to get when you bring up a minor league star.

Tommy Lasorda would have been pissed if I didn't take that opportunity.