Chris Sale for whatever reason has not been Chris Sale since September of last year. I don't know if hes hurt or his mechanics are off but for whatever reason there was been a noticeable dip in his advanced metrics to go along with an increase in the batted balls and barrel rates of his pitches. In this current baseball landscape of sabermetrics and advanced metrics sometimes it takes a rocket scientist to figure out whats going on but for Sale its pretty easy, lets take a look.
First off Sale's 4 seam fastball which sets up everything else he has including his wipe out slider had a release speed of 96.38, 97.67, 97.56, and 98.17 from June 2018 to October of 2018 and since then it has not been over 94.09 and bottomed out at 92.61 in April of this year. Low 90's 4 Seamer? That is very pedestrian especially for a Cy young type pitcher. Now what that does is it makes all of his off speed stuff look that much more hittable. If your throwing a 97 mph fast ball and then dropping an 80 mph slider the hitter has almost no chance. If all the sudden your throwing up a 92 mph heater with a 80 mph slider then the hitter is no longer afraid. What has happened as a result is Sale can no longer throw his fastball by people so he is throwing it a lot less (3% less) and relying more heavily on what has historically been his best pitch witch is that wipe out slider (4% more). What I just outlined however is that the difference in velocity no longer makes that slider as effective as it used to be so players are sitting on that fastball. In fact according to the sabermetrics his Whiff/Swing percentage on his slider went from 44% last year to 35% this year. That's crazy, not only is that a big drop but last year when he was right he was getting a whiff on his slider close to 50% of the time. To be honest its a pretty simple explanation and when that happens to a pitcher of his caliber there are only 2 reasons why, he is either hurt or his mechanics are way off (slider flattening out). The Red Sox might do him some good to place him on a quick 10 day IL stint to help him figure things out. His stuff at this present time is average and has been average since last fall.