Because of the role of mental fatigue in the development of elite soccer players, it has been a topic of interest for researchers in the last decades. First, we aim to shed light on the literature published about mental fatigue in soccer in the last 10 years. Second, based on the results obtained, we propose a new perspective on the role of cognitive effort in soccer. A systematic review (SR) was conducted following the PRISMA guidelines and registered in PROSPERO. A total of 18 articles met the inclusion criteria. The results showed an increase in the publications related to mental fatigue from 2014 onward. They were compared according to focus, sample, instruments and outcomes. Our proposal assumes that physical-physiological, technical-motor and tactical-cognitive demands entail a cognitive load that reduces the performance of players. Studies that prioritise controlling behavioural and physiological responses in cognitive tests are still needed.