AC Milan have released a statement confirming that Robinho suffered a thigh injury in Friday's 3-1 defeat to Napoli in Serie A.
As a result, the 30-year-old is expected to miss next week's last-16 Champions League first-leg tie against Spanish outfit Atletico Madrid at the San Siro.
"Robinho has sustained a muscular injury in his left thigh," read the statement. "The Brazilian attacker will undergo further tests next week."
The Brazilian international has scored once in the Champions League this season.