Pulisic has been critical for Milan this season, and any potential lay-off will no doubt prove costly for the Rossoneri.
The USMNT star is tied for the team lead in both Serie A goals and assists while his two goals in the Supercoppa Italiana helped lead Milan to that trophy earlier this month. In addition, Pulisic leads Milan in Champions League goals this season with three.
Milan are heading into a crucial point in their season, with the club currently eighth in Serie A. Their early-season struggles led to Paulo Fonseca's exit in December, and new boss Sergio Conceicao will now have a major problem on his hands with key Serie A and Champions League matches on the horizon.