Physical and mental fatigue are two sides of the same coin.
The drop off in performances is evident with both him and Cubarsi. It's simple, you need maximum 100℅ concentration and discipline in every match with such a demanding system for defenders. It's hard to keep it up when you have played so many matches and you feel so tired.
Cubarsi is a kid, Inigo maybe too old, they are both affected by this. But it doesn't mean there are no world class defenders out there who can do it.
Those should be our targets. Maybe not this summer but next. When Cubarsi develops more and grows physically he will probably be the best defender, right now he is not.