The Brazilian forward faces a spell on the sidelines due to a thigh injury he picked up in Saturday's Serie A loss to Napoli at the San Paolo, but will undergo further tests
The Brazil international picked up a thigh injury in Saturday's 3-1 defeat away to Napoli and was replaced by compatriot Kaka at half-time.
Robinho, 30, has now undergone a series of examinations to determine the extent of his injury and he seems unlikely to be match fit in time for the first leg of the Champions League round of 16 tie with Atleti at San Siro.
"Robinho has sustained a muscular injury in his left thigh," a statement on the official Milan website reads.
"The Brazilian attacker will undergo further tests next week."
The former Manchester City man has scored once in his five previous Champions League appearances this season.