He is doing the right thing by playing the John Deere. Not because it is a honorable thing to go to the place where you got your first tour win (which it is) but because much more important than the venue on which you warm up is confidence.
The Deere yields lots of birdies and making birdies feels good regardless of the course. Links golf and parkland style American golf are not as different as they are trumped up to be. Plus he got all the links prep he needs at the U.S. Open.
I would take a drop of confidence over a gallon of course prep any day.
He'll do great I am predicting a top 10 finish, there will be talk of 3 majors in a row at some point next weekend but he just won't quite get low enough.