Right, he hasn't used the change much. It isn't that good.
My point is that he has taken some speed off his slider, which used to come in at mid 90s. Now it's down in the 80s. A reliever can get by with two pitches and that slider will be effective as an offspeed pitch out of the pen. If he becomes a starter he'll need to throw the changeup a lot more. It's not a strong pitch.
So if the Reds see him as a future starter, he'll continue to work on the change as his offspeed pitch and keep the slider as a hammer.
If they see him as a closer, he'll have to have an effective offspeed pitch, which could be the slider.
I'm specutating that the drop in velocity of his slider is an indication that he has abandoned the change, which is an indication that he is staying in the pen.