Selection is planned backwards
A coach picking a cup team is looking at the following weekend as much as at the tie in front of him. Minutes are allocated across a fortnight, players carrying knocks are protected, and a squad player who needs football gets it. That is not disrespect for the competition; it is the only way to survive a congested calendar with a squad that has limits.
The public version of this conversation is always about priorities, which flattens a set of medical and physical judgements into a moral one.
Why changed teams look worse
The cost of rotation is understanding rather than ability. Football relies on habits between players who have played together: when the full-back overlaps, where the striker expects the ball, who covers a midfielder who steps out. Replace five of eleven and those habits are gone, so the team is individually competent and collectively slow.
This is why the most effective rotation changes two or three positions rather than half a side, and keeps the spine — goalkeeper, one centre-back, one central midfielder — intact.
The underdog's edge
A smaller club has no reason to rotate for a cup tie. It fields its strongest available eleven, that eleven has played together for months, and it faces a team that has been reassembled for the evening. The gap in quality remains; the gap in coordination narrows sharply.
Add a crowd and a night kick-off and you have the ordinary mechanics of a cup upset. It is rarely magic and it is usually explicable by the two team sheets.
The rotation that goes wrong
Rotation fails when it changes the shape of a team rather than its personnel. Replacing a full-back with a full-back is manageable; replacing the only midfielder who sets the tempo, or breaking up an entire defensive line at once, removes the understanding that made the side function and hands a lower-division opponent exactly the chaotic match it wants.
The better approach is partial. Keep a spine of players who have started together, refresh the positions that cover the most ground, and give the changed players the simplest version of the plan. That gets a squad through a midweek round without spending the weekend.