From what I gather the need for large athletic fullbacks is the way in which we play and opposition try to counter on us. There have been lots of times where when opposition counter they pop a cross over and Zinchenko can't defend it. I think Pep knows the PL well enough and knows how teams attack with crosses and headers. The need for Walker was for his pace and athleticism defending counters on the right while we would use Mendy to attack on the left. Alves would have been used in attack on the right because of his skills almost like a midfielder and the LB would fall back and make it like a back 3 when we lose the ball.