A 19-year-old man was sent to the emergency department following a pitch-side assessment for suspected concussion, unexplained upper abdominal tenderness and vomiting, following a high-impact tackle during a rugby match. A Focussed Assessment with Sonography for Trauma (FAST) scan performed in the emergency department suggested intra-abdominal free fluid, and subsequent head and abdominal CT imaging showed no intracranial lesion but confirmed a significant haemoperitoneum due to large splenic tear and bleeding. An emergency splenectomy was performed, which confirmed the rupture of an enlarged spleen with blood loss of almost 2 L into the peritoneal cavity. The patient made a full recovery following surgery. A subsequent histological examination revealed granulomatous inflammation characteristic of infectious mononucleosis. This unique case illustrates that physically fit patients with early hypovolaemic shock can present with symptoms mimicking concussion.