That's not how it works. Teams get detailed scouting reports on players even before they come up to MLB. What happened with most young players, is that the league discovers some sort of previously unknown weakness and starts exploiting it, but this is not WMB's case.
WMB's weakness to low-and-outside breaking balls appears in scouting reports from as early as 2011, and when he came up, he'd lay off the pitch a lot more than he did this year. That's on him, not a "league-catching-up" scenario, which is usually what happens to guys with sophomore slumps. They fall into bad habits, and take a long time to break away from them.