Pato broke down after scoring twice in yesterday's victory over Atalanta.
La Stampa says Milan patron Silvio Berlusconi immediately called vice-chairman Adriano Galliani upon hearing the news: "Tell me how he is."
But Galliani said: "The player has no extreme pain, it is something between a cramp and a strain."
Pato will undergo tests today and tomorrow. The Brazilian told reporters this morning: "I feel good. I'm feeling better."