A transfer collapsing after the medical is treated as a surprise, particularly when the player has been playing every week. The examination is not asking whether he is fit now.
The medical assesses future risk
A club is committing to wages and a fee across several years, so the examination estimates how likely the player is to remain available across that period.
Imaging can reveal joint wear, previous surgical repairs or chronic conditions that are entirely compatible with playing today and predictive of problems later.
A player can therefore be fully fit and still represent a risk the buying club decides not to price into a four-year commitment.
It is a negotiation as much as an examination
Findings rarely produce a straightforward pass or fail. More often they produce a report that gives the buyer grounds to revisit the terms.
Fees can be restructured towards appearance-based payments, contract length shortened or the deal made conditional on availability targets.
Deals described as collapsing at the medical have frequently collapsed over the renegotiation that followed it rather than over the findings themselves.
Timing makes everything worse
Medicals are often scheduled near a window deadline, leaving no room to consider an unexpected finding calmly or to seek a second opinion.
A club facing a choice between accepting an unclear report and losing the player entirely is making a decision under artificial pressure.
This is a substantial part of why late deals fail more often than those completed early in a window.
Selling clubs see the report too
A failed medical damages the player's market value and can affect his standing at his current club, since the findings become known internally.
Selling clubs therefore prefer to resolve medical questions before a deal becomes public, which is one reason quiet negotiations are the norm.
Where a player has an established injury history, the medical is expected to be difficult and the structure of the deal usually anticipates it.
Insurance sits behind the decision
Clubs insure against long-term absence, and premiums depend on the medical report, so a finding can raise the true cost of a transfer without changing the fee.
Some conditions are excluded from cover entirely, which shifts risk back onto the club and changes the calculation.
The medical is consequently a financial process with a clinical component, and understanding it that way explains most of the outcomes that look strange from outside.