Chicago Cubs: 2016 rotation was lethal and has underperformed since
One thing the Cubs have had the last few years was a really good rotation on paper. Since 2016 the Cubs rotation has consistently featured Jon Lester and Kyle Hendricks, while seeing one last season from Jake Arrieta and John Lackey in 2017 and adding Jose Quintana, Yu Darvish, Cole Hamels and Tyler Chatwood along the way. Mixed in 2017-2018 we saw stretches of Mike Montgomery starts. There have been others to make starts but these are the main guys.
Overall the rotation just has not lived up to expectations after having an excellent 2016. It has never been awful but it has not been great. In 2016 the starters were first in baseball in ERA (2.96) with a top-five FIP (3.75). The next year, their ERA went up to 4.05 and climbed to 4.18 in 2019. Note in 2018 the starters had a FIP of 4.30 (3.84 ERA), which was middle of the pack.
What also hurts is that the past few years the starters are not going nearly as deep into games, which puts strain on the bullpen. Lots of high pitch counts from not putting hitters away and giving up a lot of contact has not been a recipe for going deep.
How has it come to this? A number of reasons. Lester has been worth every penny of the deal but everyone knew the final years of said contract would likely see a natural decline due to aging. Arrieta started to decline as well in 2017 and left to get paid in Philly, while Quintana has not been the consistent guy he was in the South Side.
Come 2018, Darvish had missed most of the year due to injuries and Chatwood struggled with command his first year posting a 8.2 BB/9. Hendricks has been far and away the most consistent starter of the bunch (3.33 ERA since 2017) and Hamels had a real nice stretch from late 2018-2019 but not enough to carry the rotation during their ups and downs.
It was almost impossible to sustain a rotation ERA under 3.00 in the coming years, but expectations were still higher for what they ended up being. An overall solid but not elite rotation.