I think you have a bit too much faith in medical imaging. It’s easy enough to spot serious muscle/ligament damage but there’s loads of grey areas where it’s hard to see the injury properly and it’s almost always extremely hard to know when an injury is completely resolved, just based on scans. Most of the assessment will be functional. And that will depend on feedback from the player about the amount of pain various movements cause in their injury. So some footballers can definitely be too brave for their own good (just like some footballers can sit out more games than they need to)
I understand your point and I'm not dismissing it but my point is, using my own experiences as an example, a scan can definitively tell you whether something is a definite 'no' or a calculated risk.
You're right, there are 'grey' areas, whereby it's clear there is stress, bruising or inflammation and that may or may not be performance-impacting and/or may or may not lead to further more serious injuries. In this scenario, some players will play through the pain, others won't.
However, I am saying that if you go in for a scan on your ankle, your achilles or your knee, for example, the medical team will know if you have a break or a tear. I've had two bad knee injuries playing football and have gone through sports rehabilitation and an MRI scan on your knee can pick up hairline tears in cartilage from injuries that are years old.
So my point is, if a player DOES have an obvious tear, rupture or fracture, I highly doubt ANY club would risk that player as further impact or stress could mean career-changing injuries. As you say, it's then down to the player whether they have the desire, the appetite or the toughness to play through soreness, inflammation, bruising or stress.