As highlighted by Diario AS, the Brazilian holding midfielder is one yellow card away from a suspension that would rule him out of the crucial second leg of the tie.
Real Madrid news has been dominated by the club’s injury crisis in recent weeks – they have accumulated 41 injuries this campaign and have just 11 senior outfield players available for the trip to Bergamo.
Star forward Karim Benzema has not recovered his fitness and joins Sergio Ramos, Eden Hazard, Marcelo, Dani Carvajal, Fede Valverde, Rodrygo Goes, Alvaro Odriozola and Eder Militao on the absentees list.
Brazilian midfielder Casemiro is a near ever-present this campaign for Los Blancos with 28 appearances to his name, while he has weighed in with six goals.
There is no natural replacement for the 29-year-old in the squad, with Valverde also unavailable to replace him.