Jon Heyman of MLB Network first reported the sides were nearing a deal.
The Philadelphia Phillies made a statement when they signed Arrieta, the 2015 National League Cy Young Award winner, to a three-year, $75 million contract in 2018 after he had spent five seasons with the Cubs. While the Phils had won only 66 games the season before, their top prospects had hit the majors, and they were ready to take a big step forward.
The pitcher's fortunes since then have mirrored those of the franchise.
In 2020, the 34-year-old finished with a 4.66 FIP while averaging 6.5 strikeouts per nine innings, his worst mark since his rookie season in 2010.
That didn't coincide with a drop in velocity, either. The right-hander's four-seam fastball and sinker averaged 93.4 and 92.4 mph, per Brooks Baseball. His fastball had actually been slower in 2019 (92.8 mph), while his sinker was only marginally faster (92.9).
According to Brooks Baseball, opposing hitters slugged .277 against that pitch in 2015. That's slugging percentage and not batting average—a figure much lower than Billy Hamilton's career slugging percentage (.325).
Now let's see how hitters have slugged against Arrieta's sinker since:
- 2016: .313
- 2017: .428
- 2018: .433
- 2019: .476
- 2020: .594
That certainly qualifies as a worrying pattern.
Baseball Savant provides heat maps for where each pitch lands in the zone. Scroll down to Arrieta's, and you'll find he has been less able to elevate the ball over the years.
For years, the general thinking was that a sinker is most effective down in the zone, where hitters can do little but muster grounders. However, that rule of thumb no longer applies because position players have modified their swings to get more lift. A pitcher who works low only plays into their hands.
All this explains why Arrieta is trending in the wrong direction, and there's little reason to believe he can turn around his fortunes in a noticeable way.
His contract with the Phillies included an option of at least $20 million annually for 2021 and 2022. The drop from those terms to what he'll likely earn for the upcoming season speaks to how much his value has changed.
Arrieta can still be a good option, albeit at the back of the Cubs rotation. He may need to make some changes, though, to stave off his continued decline.
Stats courtesy of FanGraphs unless otherwise noted.