Come November Diarra is starting not an unproven/untested freshman. Nowell may even become great (and even start) but he is not where Diarra is right now. Diarra knows the offense/defensive sets, knows the coaching staff's goals, has 2 Championship rings, and was awarded the 6th man of the year award in Conference. More importantly he has the coaches and teammates trust right now.
Not sure what Nowell brings initially to match that unless posters are relying on high school film. Nowell will have time to acclimate, study and learn, ain't nothing wrong with a freshman doing that.