First off, I'm not sure how Joseph helps as a secondary playmaker but Ferrell doesn't. Why do you think that way when the majority of the stats point towards not only Ferrell being a better offensive player but them being on the same level from an assist & turnover perspective?
The impact stats over the past few years have jumped around on these two. I'm not sure how reliable it is to reference last year when we see huge swings like this:
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Joseph killed it this year across impacts stats, but was bad last year. Ferrell killed it last year across impact stats, but was bad last year. I think any reasonable person would look at this data and say its hard to distinguish who's undoubtedly better. I took a basic average at the bottom for the 3 seasons to give us perhaps a better sense for how these two perhaps perform at their norm. There's not much difference between the two as you can see. Their total impact stats are very close across the board.
I think the other thing to keep in mind when looking at some of the impact stats is that Joseph played 67% of his minutes this year with Sabonis on the floor. Sabonis was a legit MIP candidate this year who went from (per36)...
.567 TS% / .514 FG% / .351 3PT% / .750 FT% / 17.1 PPG / 11.4 RPG / 3.0 APG / 0.8 SPG / 0.6 BPG / 2.7 TOPG
to...
.630 TS% / .590 FG% / .529 3PT% / .715 FT% / 20.4 PPG / 13.5 RPG / 4.2 APG / 0.9 SPG / 0.6 BPG / 3.1 TOPG
I wouldn't be surprised if some of Sabonis impact is rolling into Joseph's metrics this season (especially when we look at how lackluster these metrics were in 2016-17 & 2017-18).
Look I'm not saying it wouldn't be helpful to have a better backup PG. I'm just not sure Joseph is going to be leaps & bounds better than Ferrell. The fact that I'm saying that statement while knowing Joseph makes $9.3 mil more per season than Ferrell, makes me cringe at this signing.