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Background/Objectives: Diagnosing anterior cruciate ligament (ACL) insufficiency, particularly partial tears, remains challenging with standard static imaging. This study introduces a novel conceptual approach: assessing the dynamic kinematics of the infrapatellar fat pad (IPFP) as an indirect marker of ACL deficiency. Methods: In this biomechanical proof-of-concept study, dynamic ultrasound tracked IPFP kinematics in eight cadaveric knees evaluated in intact (Control), Sham, and Torn ACL states during passive flexion. The primary endpoints were (i) the absolute anteroposterior position at 90° (y₂ − y₁) and (ii) the posterior displacement during the 60–90° flexion arc (ΔY 60–90°). Results: ACL deficiency significantly altered deep-flexion IPFP kinematics. For ΔY 60–90°, the Torn ACL group demonstrated a substantial loss of posterior excursion compared to the Control group (Mean ± SD: −0.25 ± 1.03 vs. 2.88 ± 1.29 mm; Welch’s p < 0.001; Hedges’ g = 2.54, 95% CI: 1.18 to 3.89) and the Sham group (3.46 ± 1.63 mm; p < 0.001; g = −2.57, 95% CI: −3.90 to −1.25). Consequently, for y₂ − y₁ at 90°, the Torn ACL group remained abnormally anterior versus Control (p = 0.003; g = −1.97) and Sham (p < 0.001; g = −1.82). Conclusions: ACL deficiency induces a distinct reduction in posterior IPFP displacement. While these massive effect sizes establish a strong biomechanical rationale, this study serves as a foundational proof-of-concept. Large-scale in vivo clinical trials are strictly required to validate its diagnostic utility before clinical implementation. Full article