Those putting forward the opposite view aren't saying he never could adapt - just that it's unlikely in the rapid time frame, and there could be other keepers available who could adapt quicker or are already at a level Pep wants for this specific needs.
The other issue put forward isn't that he has no control, or can't do keepy uppy / crossbar challenge, it's about doing it with Suarez, Neymar and Messi closing you down! or even the likes of Costa and Hazard.
Ultimately, by Pep's way of playing - can the keeper turn the ball around very rapidly so that we can turn being under pressure into a counter attack? If we can't do that quickly and the keeper picks up the ball and takes too long to decide, the moment is lost - and the very top teams will be back in position. Schmeichel used to throw it out with considerable speed and accuracy at one time - which was along the same lines. It was devastatingly effective.