Since the American League implemented the designated hitter in 1973, the role has been a consistent source of both controversy and offensive firepower.
Whatever one thinks of designated hitters, they hit. It's right there in the name. And indeed, AL DHs posted a collective .769 OPS in 2015 and .780 in '16, to rank second and first, respectively, among all positions in those seasons.
In that context, what happened in 2017 is jarring. AL DHs produced a mere .735 OPS, putting them just behind shortstops (.736) and ahead of only catchers (.716) for eighth among AL positions (excluding pitcher).
Meanwhile, as a whole, AL hitters generated an OPS of .753, putting DHs 18 points below that mark, the largest negative difference in history. In fact, only once before since the position was created have AL DHs underperformed the league average OPS. In contrast, AL DHs outperformed the league average by 39 points in 2015 and 36 points in '16.
Lowest AL DH OPS, relative to overall AL OPS

1. -18 points -- 2017 (.735 vs. 753 overall)
   
2. -12 points -- 1985 (.721 vs. .733 overall)
   
3. +1 point -- 2013 (.726 vs. .725 overall)
   
4. +4 points -- 1989 (.713 vs. .709 overall)
   
5. +4 points -- 1974 (.698 vs. .694 overall)

Among the 15 AL clubs, only the Nelson Cruz-led Mariners and Edwin Encarnacion-led Indians posted an OPS of .760 or better from the DH spot. Six teams, including the World Series champion Astros, came in below the .700 plateau.
So why didn't designated hitters, you know, hit? Here is a look at three causes for this drastic dropoff:
The Big Papi Factor
When a legendary hitter still in peak form hangs up his lumber, his absence is going to create a void.
David Ortiz spent the vast majority of his career at DH, where he posted a .942 OPS. Even at age 40 in 2016, his 1.033 OPS as a DH ranked fourth all-time for a single season (minimum 600 plate appearances), and second for Ortiz personally behind his 1.080 mark in '07.
With Ortiz gone, the Red Sox turned over the DH reins mostly to Hanley Ramirez, who had a solid but un-Papi-like .771 OPS in 474 plate appearances in that role. Another 118 plate appearances went to Chris Young and Dustin Pedroia, who combined for a .547 mark. This dropoff might help explain Boston's reported interest in free-agent slugger J.D. Martinez.

Veteran blues
This past season was just the fourth in which there were as many as four players who posted a sub-.700 OPS as a DH in at least 300 plate appearances. In contrast, there was just one such player in each of the previous four years to do so.
• In his 20th and final big league season, 40-year-old Carlos Beltran produced a .670 OPS in 448 DH plate appearances for the Astros.
• Battling health issues in his age-38 season, Victor Martinez notched a .694 OPS in 432 plate appearances as the Tigers' DH.
• The Angels' Jose Pujols put up by far the weakest rate stats of his Hall of Fame career, including a .678 OPS in 611 DH plate appearances in his age-37 season.
• Coming off a stellar offensive campaign in 2016, 31-year-old Mark Trumbo returned to the Orioles only to generate a .635 OPS in his 467 plate appearances as a DH.
Combined, these four highly accomplished hitters lost an average of 131 points off the OPS they produced in the DH role in 2016.

Role reversal
Both Lucas Duda and Miguel Sano swung the bat effectively in 2017 -- but only when they played the field.
In 92 games as a first baseman, Duda slashed .242/.342/.572 (.914 OPS), including .225/.326/.675 (1.001 OPS) after the Mets traded him to the Rays in late July. But in 24 games as a DH -- all with Tampa Bay -- the left-handed slugger managed a .136/.240/.250 line (.490 OPS).
Sano raked as a DH as a rookie in 2015 (.908 OPS) but has struggled in that role since. This past season, he generated just a .182/.294/.261 line in 25 games as the Twins' DH while slashing a robust .287/.369/.575 (.944 OPS) at the corner infield spots.
Baseball-Reference's tOPS+ stat compares a player's OPS for a particular split to his overall OPS, with a tOPS+ of less than 100 indicating that the player performed worse in that split. As DHs in 2017, Duda and Sano had respective tOPS+ of 25 and 35 -- two of the four lowest in history for a player with at least 100 DH plate appearances in a season. Houston's Evan Gattis (55) also ranked 21st on that list.
These are small samples, so that doesn't mean either player can't perform at DH. However, the drastic 2017 splits of Duda, Sano, Gattis and some others certainly contributed to the plunge in overall DH performance.

Conclusions
While AL DHs saw their OPS plummet 45 points from 2016 to '17, that drop can be explained almost entirely by the combination of that quartet of Beltran, Trumbo, Martinez and Pujojs struggling and Ortiz's retirement.
Take the 2017 OPS of .735, and replace Beltran, Martinez, Pujols and Trumbo's DH splits with their splits from '16. Then replace Ramirez's '17 splits with Ortiz's from the year before. The result is a line nearly identical to what AL DHs produced in '16.
2016 AL DHs: .328 OBP, .452 SLG, .780 OPS
2017 adjusted: .329 OBP, .448 SLG, .777 OPS
There's no sign that Big Papi is coming back, but there still is reason to think that the DH decline of 2017 will turn out to be a blip. It's not just that established hitters could bounce back, or simply produce more favorable splits. Here are four other potential avenues for improvement:
• As mentioned, the best hitter on the free-agent market, J.D. Martinez, remains available. While he has mostly played the outfield throughout his career, his poor defensive numbers could make him a DH in 2018, depending on his destination.
• Shohei Ohtani was a two-way sensation with the Nippon-Ham Fighters, but it remains to be seen how his eye-popping offensive numbers will translate -- if the Angels even give him an extended opportunity at the plate. Still, an Ohtani/Pujols DH combo is an intriguing thought.
• Khris Davis has developed into one of the game's top sluggers in Oakland. His .886 OPS as a DH from 2016-17 trailed only Ortiz and Cruz (minimum 300 plate appearances), and his time in that role could increase dramatically after the A's traded Ryon Healy to Seattle earlier this offseason.
• Giancarlo Stanton only got to DH in the occasional Interleague game with the Marlins but had no trouble doing it, batting .333/.390/.630 with four home runs in 59 plate appearances. Now, with Stanton joining a crowded New York outfield, either he or Aaron Judge could fill that spot in the lineup in most of the club's games.