We report the case of a 59-year-old man with severe knee pain and inability to flex his toes or invert his plantar flexed foot after an external rotation injury to his knee. MRI showed rupture of the popliteus with a haematoma compressing the neurovascular bundle in the proximal calf, and electromyography demonstrated signs of an axonotmesis of the posterior tibial nerve. There was progressive nerve recovery over 24 weeks. Isolated rupture of the popliteus should be considered in any patient with an acute haemarthrosis, lateral tenderness and a stable knee, especially after an external rotation injury.