Serious football injuries are 10 times more likely to happen during match play than during training sessions. The most common injury in football is a knee injury and this can also be the most complicated with the likelihood of a long layoff or surgery.

If you do not have a physiotherapist involved within your football team aim to have a first aider. You will need someone who knows how they will assess and deal with an injury before it happens. The FA website gives details on how you can get First Aid training in your area. The FA First Aid Training

How does the knee injury occur?
Most knee injuries in football occur due to a sudden twisting movement, from a sudden change of direction, landing awkwardly after jumping up to head a ball, or from an opponent’s challenge blocking the leg as the rest of the body twists around.
Knee injuries can be made worse if there is a mismatch between footwear and the playing surface i.e. blades caught in the playing surface as the knee twists round.
Knee Ijuries
What to do when a knee injury occurs.
If the injury occurs during training players are likely to stop straight away. However the adrenaline rush during a match is likely to mean they try to carry on. If a player is limping, insist they leave the pitch for assessment. The knee needs to withstand enormous strains during football, so unless it has a full range of pain-free movement, players should not continue, otherwise they risk a second more serious injury.
Serious knee injuries need direct action from the beginning. If a player cannot bear weight, feels their knee is wobbly or if the joint is rapidly swelling the knee should be protected from further injury by being splinted. Immobilise the knee on the pitch, stretcher the player off and arrange a transfer to the local casualty department for assessment. Let common sense prevail, even if the wait in casualty in a long one, it is usually worth it.
The signs of a serious knee injury are:
Unable to bear weight
Unable to fully straighten the knee.
Rapid swelling of the knee joint
Severe pain
Players heard a pop or felt something give way.

Knee injuries can be quite complex and often require physiotherapy before a return to playing. Players should not rush back too soon to avoid a repeat of the injury. The use of knee supports will give some extra protection and support to the knee once a player returns to fitness.
Knee Supports