The Corey Kluber trade rumors haven't dissipated in the new year. The Padres and Dodgers are still involved, as MLB.com's Jon Paul Morosi reported late Friday night.
Obviously, the Indians ace would be highly coveted if he is available. But will Kluber be able to maintain his elite performance in 2019 and beyond -- whether it's in Cleveland, San Diego or Los Angeles?
It's a more interesting question than you might think. For one thing, Kluber will have to deal with the same age-related issues as any starting pitcher entering his mid-30s -- velocity, for example. Kluber's average sinker velocity has dipped steadily: from 93.8 mph in his Cy Young Award-winning 2014 season, to 93.5 mph in '15, 93.1 mph in '16, 92.5 mph in '17 and 92 mph in '18. But maybe he can pitch around that -- hitters' results against Kluber's sinker haven't changed much over that timespan, regardless of the velo.
The big question has to do with Kluber's trademark pitch -- his curveball, widely regarded as one of MLB's best offerings.
On the surface, nothing looks alarming. Here are the year-to-year results against Kluber's breaking ball, looking at weighted on-base average, which measures the pitch's overall effectiveness. Kluber's curve has ranked in the top 5 among starting pitchers every year, and has been the best by wOBA two years in a row.
Hitter wOBA vs. Kluber's curveball, 2014-18
(With MLB rank among starters)
2014: .112 (1st)
2015: .142 (4th)
2016: .130 (3rd)
2017: .111 (1st)
2018: .143 (1st)
But underlying those results, there are some numbers that point to a concern: Kluber's curveball, his best pitch, didn't have the same swing-and-miss dominance in 2018 that it did in his Cy Young Award-winning season in 2017.
From 2017-18, Kluber's whiff rate on his curveball dropped more than 10 percent. In '17, when hitters swung at his curve, they missed 49 percent of the time. In '18, they missed 37.7 percent of the time. That's not bad, but it's closer to his 2015 season, when he had a 3.49 ERA, than his 2017 season, when he had a 2.25 ERA.
Kluber's curveball whiff rate, 2014-18
2014: 44.5%
2015: 39.0%
2016: 50.6%
2017: 49.0%
2018: 37.7%
His strikeout rate with his curveball dropped by more than 15 percent -- from 54.7 percent in 2017 to 39.3 percent in '18. It was the first time of any season, going back to 2014, that fewer than half the plate appearances ending on Kluber's curve were strikeouts.

So what happened in 2018? One possible explanation is the break Kluber was getting. According to Brooks Baseball's pitch movement numbers, over the years from 2014-17, the type of movement on Kluber's curveball had been growing steadily more vertical, and less horizontal. By 2017, it was getting more drop, and less horizontal run, than in any prior year. But in 2018, the numbers jumped back in the other direction -- less drop, more run. That could have made a difference when it came to missing bats.
And it might have contributed to another change: Kluber's curveball usage rate. He'd used his breaking ball more often than ever before in 2017 -- a conscious effort to take advantage of his greatest weapon, as MLB.com's Jordan Bastian reported during the season. But last year, he didn't maintain that trend.
Kluber's curveball usage rate dropped from 28.1 percent to 22.5 percent -- and in two-strike counts specifically, he used it even less, down from 41.9 percent to 31.9 percent. Instead, he often went to his cutter, especially against left-handed hitters.

Maybe Kluber has some work to do in 2019 to find his "A" curveball. Maybe 2017 was just an exceptional season, even for him. But here's the good news -- Kluber can still be one of the top pitchers in the game. Even with less swing-and-miss on the breaking ball, Kluber's overall metrics for 2018 were generally in line with what they usually are.
Look at Kluber's expected wOBA -- the Statcast™ metric that shows the quality of contact a pitcher is allowing, based on the exit velocity and launch angle of batted balls against him. Kluber's xwOBA in 2018 was .277, ranking among the top handful of starters -- and actually better than both his 2015 and '16 marks.
Kluber's xwOBA by season
Since Statcast™'s intro in 2015
2015: .281
2016: .280
2017: .252
2018: .277
And he's still a master of his craft. Kluber remains one of the best starters in the Majors at getting hitters to chase pitches outside the strike zone. His chase rate induced was 33.6 percent in 2018, ranking fifth-best among regular starters -- and essentially the same as it has been every year.
Kluber's chase rate induced, 2014-18
2014: 33.4%
2015: 34.3%
2016: 32.1%
2017: 33.4%
2018: 33.6%
And he's still striking batters out without walking many. Kluber's 26.4 percent strikeout rate was a dip from his 34.1 percent in 2017, but right around the same as his 28.3 percent in '14, 27.7 percent in '15 and 26.4 percent in '16. His walk rate was as low as ever -- actually, lower, a career-best 4.0 percent.
So what should Kluber's team expect out of him this season? Well, they should keep an eye on the curve, but all in all, they'll probably get more ace-level performance.