Contusion or Bruise of the Knee: This is the most likely diagnosis given the mechanism of injury (being kicked in the knee). The impact would likely cause bleeding into the soft tissues, leading to a bruise.

Hemarthrosis (Knee Hematoma): A direct blow to the knee can cause bleeding into the knee joint, leading to hemarthrosis. This condition can be painful and may limit knee mobility.
Ligament Sprain (e.g., MCL, LCL): The force from the kick could potentially stretch or tear the ligaments around the knee, such as the medial collateral ligament (MCL) or lateral collateral ligament (LCL), especially if the knee was twisted or bent at the time of impact.
Meniscal Tear: Although less common from a direct blow, if the force was significant and the knee was in a position that put stress on the menisci, a tear could occur.

Fracture (e.g., Tibial Plateau, Patella): While less likely than soft tissue injuries, fractures can occur from significant direct trauma to the knee. Missing a fracture could lead to improper healing and long-term disability.
Vascular Injury: A severe injury could potentially damage the blood vessels around the knee, leading to complications such as compartment syndrome or significant blood loss.
Neurological Injury: Direct trauma to the knee could also injure the nerves, such as the peroneal nerve, which could result in numbness, weakness, or paralysis of the affected leg.

Osteochondral Defect: A piece of cartilage and bone can become damaged and detached from the end of the bone due to the trauma, which might require surgical intervention.
Dislocation of the Knee: Although rare, a severe force could dislocate the knee joint, which is a medical emergency due to the risk of vascular and neurological complications.
Infection (e.g., Septic Arthritis): If the skin was broken during the injury, there's a small risk of introducing bacteria into the joint, leading to septic arthritis, a serious condition requiring prompt antibiotic treatment.