Coaches need to learn how to get a new squad in preparation in case of an injury or even illness because there is nothing more that can destabilize the performance of a club than these things, and this is why you need to have a strong substitution as a club and everything is going to meet their expectations every time.
For a club to have such a strong backup substitute as you're describing the club must be fiscally sound and by that I mean they must have financial stability enough to cover for that, which currently Manchester United are not due to some financial crisis the club management is passing through. Club like Real Madrid that are known to have the kind of bench you're making insight about is as a result of their financial capacity over the years till date, it was same for Man. United until recently when the club started facing fiscal crisis making them not to able to afford the best of players in Europe.