A 28-year-old male footballer presents to the orthopaedic clinic with a 3-week history of left knee pain and swelling following a non-contact injury during a match. He reports a sudden 'popping' sensation in the knee while pivoting on a planted foot. On examination, there is joint effusion, and the anterior drawer test is positive at 90° knee flexion. Lachman test (at 30° flexion) is also positive. McMurray's test is negative. Which structure is most likely injured?