Let's get something straight first - Daniel Murphy is a good hitter. He has a career 109 wRC+, and has a 110 wRC+ in each of the past two years. He is above average, but he is not that far above average. His ridiculous postseason performance before the World Series was bound to crash at some point regardless of what anyone did or did not do; the question was when. Would it be this year? Next year? It turns out the Royals were when Murphy hit the wall.
In general, Murphy is a pretty solid hitter in terms of what he does when he makes contact. He hits a good amount of line drives, sprays the ball around the field, and doesn't strike out much. His .278 BABIP in 2015 was pretty low compared to his career value, but he hit for more power this year and put the ball in the air more. It makes sense.
According to FanGraphs' pitch type linear weights, Murphy did well against the sinker and slider and struggled against other pitch types. The curveball and changeup gave him the most trouble. The point of this background information is to give context to how teams approached Murphy during the regular season, during the NLDS and NLCS, and during the World Series.
Not surprisingly, the strategy against Murphy in the regular season was simple. Throw pitches low and away (Murphy is a lefty). Ignore all other parts of the known universe. That same strategy took place during the playoffs, but a few more pitches than normal were left not low or not away. Murphy punished those pitches.
Interestingly, the Royals added up and away as another area of concentration.
Looking at the up and away area over Murphy's career, there might be a few reasons why the Royals chose to add in that area. While Murphy can still hit the ball there, it's not as likely to be solid contact as any pitch inside. He's more likely to whiff and pop up against those up and away pitches than the down and away ones (at least within the zone - outside the zone is a different story).
The results are plum-surprising. Murphy hit seven homers in the NLDS and NLCS; he hit zero against the Royals. Murphy struck out only six times in the NLDS and NLCS, never more than once per game; Murphy struck out seven times against the Royals, twice in three separate games. Five of Murphy's six walks came against the Royals, so Murphy just wasn't buying what the Royals were selling.
The Royals also adjusted their pitch mix. According to Brooks Baseball, during the regular season Murphy saw 63 percent hard stuff, 23 percent breaking stuff, and 14 percent offspeed stuff. He saw more hard stuff and less breaking stuff in the NLDS and NLCS, but it was not a big swing.
Then the World Series happened. Murphy saw a drastic decrease in hard stuff, down to 50.7 percent, and an increase to 24.6 percent each in breaking and offspeed stuff.
In addition to the impending regression, my thought is that the Royals were able to find a pitch mix that was a little less predictable. They sacrificed strikes for that unpredictability, but during the short sample of World Series games Murphy was unable to adjust to the increased breaking/offspeed stuff and more varied location. Murphy did not make much solid contact that I remembered. Murphy will be remembered much more for his defensive miscues, but let's not forget about the lack of impact his bat had.