Robinho to miss Atletico match
The Brazilian player hurt his thigh during Saturday's 3-1 Serie A defeat at Napoli and was replaced by Kaka at half-time.
A statement from Milan read: "Robinho has sustained a muscular injury in his left thigh.
"The Brazilian striker will undergo further tests next week."
The 30-year-old has scored three goals in 18 league appearances for the Rossoneri this season.
Milan will be hoping Robinho can recover in time for the return leg against Atletico at the Vicente Calderon on March 11.
However, Atletico have problems of their own, with midfielder Tiago Mendes also out of the game.
Tiago was stretchered off during Saturday's Primera Liga game against Almeria after a late challenge from compatriot Helder Barbosa.
Tests on the Portugal international revealed a fractured wrist and a sprained ligament in his right knee, Atletico confirmed.
Tiago will undergo surgery on his wrist on Tuesday and is expected to miss his team's next few games.