When a hitter's skills begin to erode, an easy place to start is to look at his plate discipline. Ortiz, however, hasn't had trouble making contact, with a whiff percentage of 15.8, which is equal to what he had in 2014 and better than both his career mark and the American League as a whole. He's walking slightly less than usual, but that 11.3 percent mark is a top-15 mark in the AL. So that's not the problem.
Nor is the problem hitting right-handed pitching, because Ortiz has been outstanding there. Against righty pitching, he has a 154 wRC+ (weighted runs created plus), which is to say, "54 percent better than league average," and better than Kris Bryant or Josh Donaldson. Last year, that was 131. For his career, it's 159. He's always crushed righties, and he's continued to crush righties. Perhaps unsurprisingly, his outstanding 94.22 mph exit velocity against righty pitching ranks seventh overall among those with 50 plate appearances.
Against lefty pitching, however, it's been a different story. In 75 plate appearances against southpaws, Ortiz has just eight hits. That's a .111 batting average, a .138 batting average on balls in play, and a -40 wRC+. It's unfathomably, unthinkably ineffective, and while 75 dispersed plate appearances aren't enough of a sample size to declare him completely unable to hit lefties after so many years of being able to do so, it's also been bad enough that it's dragged his overall line down entirely.
With a line like that, you'd probably expect that he's getting dominated by his fellow southpaws. But that's not exactly what's happened, because Ortiz is still hitting the ball quite hard against lefties. Fifty-two lefties have had at least 20 plate appearances against other lefties -- we drop the threshold from 50 plate appearances simply because there are fewer lefty pitchers around -- and only 7 have had a better exit velocity than Ortiz' 90.16 mph, a number that includes sluggers Bryce Harper, Prince Fielder, and Freddie Freeman.
So if he's still hitting it hard, what's causing his issue? It's not really striking out, because while his whiff rate is up over 2014, it's still below his career average. It's not the shift, because he's always been shifted against for most of the last decade.
With an unsustainable .138 BABIP, part of it is simply poor batted ball luck, but it might also be about Ortiz finally getting sick of hitting into the shift and attempting to do something about it. Take, for example, this batted ball spray chart, which shows a clear difference in his grounders, liners, and flies against lefties from the last two seasons:
We see that 62.1 percent of Ortiz's batted balls against lefties are going to center or left, well up from the 54.2 percent it was last year. It's an admirable attempt to go to the opposite field and stop hitting into the shift, but it might be counter-productive. (The devil's advocate view would argue that his bat speed has slowed, though his production against righties and overall solid batted ball velocity would stand against that.) The problem is that Ortiz is putting up a career-low 31 percent fly ball rate against lefties, because he's trying to move away from his power field. If he's not hitting flies, he's not hitting homers, and he's not the type of runner who beats out that many grounders.
Whether or not he sorts out the lefty problem, we're already seeing the signs of life that the batted ball data always suggested. Over the weekend, Ortiz hit homers off of R.A. Dickey and Marco Estrada. In June, that wRC+ is up to an excellent 138. It's not unreasonable to suggest that with the way the Red Sox roster is put together, Ortiz shouldn't be earning starts against lefty pitchers anyway. (Any opportunity to get Hanley Ramirez off the field and into the DH role would be a welcome one.) But it's far too premature to suggest he's done. There's still plenty of life in that bat.