My guess: Pep realised his players' deficiencies - we couldn't hold the ball well enough and conceded too much counters. If you do that too often, it becomes impossible to use energitic pressing as you're quickly running out of steam (remember how poorly we played in second halves at the start of the season?).
Now we seem to be fixing this problem - B.Silva's acquisition is the most telling (and most needed) transfer for me. Possible adding of Alexis, who is a good ball-retainer, also confirms that direction of our tactical development.
As for FBs: the adding of powerful athletic fullbacks is a must [especially] for high-line, pressing-based system as they'd need to cover vast spaces while pressing. I wouldn'be surprised to see Mendy in a 'hybrid' CB/LCB role if he signs for us.
We may disagree, but it's always pleasant to read an educated response ;)