From Jun 2018:
Hip surgery is no joke. That’s what Kalil underwent in September 2016 to fix an issue that began bothering him in high school. The pain sharpened his third season in Minnesota, and his only recourse two years later was to go under the knife.
“I’ve had knee scopes here and there, but I never had a major surgery like that,” Kalil said. “So I thought it was just, ‘OK, the problem's fixed, now I'll be good and it won't hurt.’”
And it didn’t anymore.
But as surgery corrected one problem, it created others.
The longer muscles function a certain way, the longer it takes them to adjust to change. Kalil's bad hip had limited his bend for about 10 years. So when surgery fixed the hip – where so much of the body's anatomy is connected – the muscles around it needed time to learn a new range of motion.
“I was struggling in OTAs because I couldn't really move that well yet,” he said. “My groin was having issues because of the new mobility. Probably midway in camp is when I started feeling [better].”
For a guy whose athleticism made him a No. 4 overall pick, struggling to move was a significant problem. It didn’t help that it took him a bit to learn a new system.
“Obviously, you saw during the season the first few games I was kind of just iffy and a little behind,” Kalil admitted.
Added Rivera: “Early on he was stiff.”
According to Pro Football Focus, Kalil allowed 4.0 sacks in the first three weeks of his first season in Carolina. But while sacks aren't the only relevant measurement for a pass blocker – and rarely do we know whose assignment was whose on any given play – Kalil gave up just 3.5 sacks over his final 14 games, including the Wild Card visit to New Orleans.
https://www.panthers.com/news/matt-kalil-night-and-day-ahead-of-last-spring-20714874
That article was optimistic, but he then missed the entire 2018 due to knee soreness:
Kalil experienced knee soreness during training camp but played in the first two preseason games. A few days after the second game against the Dolphins, Kalil consulted with Dr. James Andrews in Florida and underwent an arthroscopic procedure.
https://www.panthers.com/news/matt-kalil-placed-on-injured-reserve
He basically hasn't been good since the hip surgery. He got a 50 grade in 2017.