Just give them a chance!
Among the first half's most terribly disappointing players were Ryan Zimmerman, Carlos Beltran, Carlos Gonzalez, and especially Robinson Cano and Matt Kemp. Also, Michael Brantley was looking like a one-season wonder.
Well, here's where those guys rank in wRC+ since the All-Star Game, among qualifying major leaguers:
7th Brantley
9th Gonzalez
19th Cano
22nd Beltran
26th Kemp
Zimmerman hasn't played enough to qualify, but he's batted .311/.352/.672 since the break.
All of which is only to suggest (again) that we shouldn't get too worked up over a player's statistics for a month, or even three months. If he's healthy he'll probably wind up doing about what we expected.
Which is not to suggest that everybody will hit, always. Early-season disappointments like Pablo Sandoval and Adam LaRoche and Yadier Molina and Hanley Ramirez and Melky Cabrera are still disappointing, and might always be disappointing.
Then again, with the exception of Ramirez, those guys didn't enter 2015 with the same histories of hitting success as those in the earlier group. We expected goodness from the second group, but not greatness. Even if the Red Sox are paying for greatness...