I think the relevant point is it's taken a number of months for players Forest have signed from the Bundesliga to get up to the speed in the EPL.
You could argue it took Mangala a season. It took Awoniyi long enough for the board to sack the recruitment team which signed him, Niakhate tore his hamstring very early on.
If we're signing cover for at No 10, we need someone who could slot straight in almost seamlessly, not someone who needs a few months of playing in the EPL to settle in.