Personally I don't think Mangala is a Pep defender and I'm okay with replacing him with Stones, he often loses focus, makes some very questionable decisions and his passing is mediocre, task him with building from the back and put him in an even higher line than Pelle's and we'll see a nightmare unfold. He's got all the physical attributes to succeed but you can't give him a better footballing brain and unfortunately that's where he's lacking, he's had two seasons that can be called average at best as he constantly switches between being fantastic or absolute garbage, if there's any time to try and cut our losses it's now.
Stones is far from perfect himself but he's only turning 22 in a few days and can be moulded into becoming a fantastic all-around defender.