He was in pretty steady decline, but the curve went off a cliff.
His drop from 2011 to 2012 was something that should have been alarming (.909 to .789), and the fact that he did not immediately go up in any of the following years showed there was cause. The expectation should have been for further decline- possibly as large as the 2011 to 2012 one.
.909> .789> .758> .739 right before Boston.
Looking at his last two years, the drop off was just 20-30 points a year, but even if he continued at that rate, we might have expected...
2015: .715
2016: .690
2017: .665
2018: .640
2019: .615
If you took the average yearly drop from 2011 to 2014, it would be 170 points divided by 3 or about 55 points a year:
2015: .685
2016: .630
2017: .575
2018: .520
2019: .465
Somewhere in between could have been expected, and so far, somewhere in between is what we got.