Atletico Madrid will be without Javi Manquillo for both Champions League legs against Milan.
The 19-year-old fell on to his neck during the 2-0 Copa del Rey semi-final defeat to Real Madrid last night.
Tests have now shown the defender suffered fractured vertebral lamina and will have to wear a collar for four to six weeks.
Atleti are already without the injured midfielder Tiago for this Round of 16 tie.
The first leg is at San Siro next Wednesday with the return in Madrid on March 11.