The biggest hope is that he'll be able to fill that gaping hole between the right fullback and the right central midfielder in the build-up. It's a crucial role in our current tactics. After all, one of his major strengths is that he can pick a pass. The star defender, Varane, for all his other qualities, often looked lost at sea in that role (which, i guess, played its part in the rumoured issues between him and ETH). Maguire is too slow on the ball, while the others (Lindelof, Evans etc.) are more miss than hit.
On the other hand, the elation of signing a good player in a position we needed strengthening shouldn't make us shrug off all criticism. What he hasn't done yet, both in Turin and Munich, is to convince the Italians that he can pick up the mantle from the ageing Chiellini/Bonucci and the Germans that he can be relied on to build a defence around him. Both clubs paid a premium price to get just that. Also, if i'm not mistaken, his flourishing at Ajax coincided with the more experienced Blind partnering him at the back and organizing the defence.
The good news is that INEOS didn't pay for that. But the question as to who will lead the defence still remains unanswered. Hopefully, it will be a fit Martinez, but if injuries strike again, we may find ourselves between a rock and a hard place.