I was amazed we didn't buy a centre half in the summer, and in truth we're overly reliant on 3 or 4 individuals in as much as we have no-one else in reserve capable of doing what they do to the same level and whose absence significantly weakens us. Stones is one of them, Fernandinho, KDB and Sane are the other 3.
Vinny is not the player he was, but I think we'll be ok with him until Stones gets back. The bigger concern is what happens if Vinny gets injured again as well and we have to rely on Mangala, as the latter simply isn't a good enough player in possession to cope with any kind of pressure. There were 2 or 3 times against Feyenoord when balls were pinged to him at knee height and I just about cacked my pants as he gawped down at the ball desperately trying to get it under control, blissfully unaware of opponents closing in, and ended up playing hasty passes as a result. Plus his presence completely robs us of momentum. Stones and Otamendi are probably the two best centre halves in the Premier League at carrying the ball up over halfway and playing quick accurate passes. With Mangala everything slows down whilst he gets the ball under complete control, nearly all the passes go sideways, safety first, and he doesn't carry the ball forward with any conviction. It's a potential problem for sure