That's probably a better question to ask with a follow up of, "How much more have they improved from 2017, because of JD?" (if any)
Some seem to think it is the "Papi effect." I'm not sure Papi had hitters emulating him or his style, or that he helped others in a hitting coach way all that much (more, less or the same as JD?) Some felt his "protection was the main reason others did better, and his drop off after losing Manny added some evidence to this claim (although JBay did pretty darn well for a year and a half.)
It's hard to assign percentages to all the factors involved in the turn around of almost all our players not named Vazquez. I was positive most would have better seasons before we even signed JD. These are probably the biggest factors in my view (not in any order of influence:
1. Better health (We had a lot of players playing while hurt last year, especially Betts, Bogey & Moreland.)
2. Players getting closer to prime or with one more year of added experience (Beni, Betts, Bogey).
3. New coaching and manager.
4. JD added into the line-up.
5. JD added as mentor and coach.
6. Osmosis (Players just seem to do better when everyone else around them is doing better.)
7. Voodoo magic (Just kidding.)