Lance was healthy in 2015 Keuchel had back to back great seasons. In 2013 (a bad year) he posted a 3.63SIERA and 3.58 xFIP (which are both solid). In 2016 (another bad year) he posted a SIERA of 3.77 and an xFIP of 3.53 (Solid again). The advanced stats liked Keuchel for the last 4 years. That isn't based on hope, but rather advanced data that shows Keuchel should be much better. The rotation was 2nd in the AL for ERA in 2015 and 8th last year. That is arguably our best possible outcome and our worst.
The issue with McCullers (outside of the injury) is his innings pitched. They tried to limit it in '15 to have him available for the playoffs, and he wound up pitching only 125 innings (although this was the most he'd ever done). Last year his injury limited his innings pitched considerably, and he wound up only pitching 81 innings. So there are still question marks for him going into next year (is he recovered from the injury and how many innings can he pitch next year before he hits a wall and if too many innings does he start having issues again)? So there are still big question marks on McCullers moving forward. That's why they more then likely keep Fiers and why they traded for Morton.
