Andrea Conti suffered a setback on his return from injury, with the AC Milan right-back spraining the same knee that required surgery in September.
Conti, who arrived from Atalanta last summer in a reported €23-million deal, had just rejoined his teammates in full training after a months-long recovery process.
Initial scans on the 24-year-old's knee on Tuesday excluded another ACL tear, but confirmed a "severe sprain" to the lateral knee compartment. Furthers tests will be conducted in the coming days to determine the severity of the injury.
Milan has struggled to stay healthy in the right-back position all season, with Ignazio Abate and Davide Calabria also missing time at various intervals.
Conti appeared to be in contention to start against Juventus on Saturday, but hopes of a showing in Turin have now been dashed.
He has made just five appearances for the Rossoneri in all competitions.
