The official club website confirmed that Pato has suffered "a first to second degree lesion in his right thigh.
"The player will undergo a course of physiotherapy over the next few weeks and ulterior tests."
To make matters worse, defensive rock Nesta "has a large bruise on the muscle of his right thigh. The player will undergo further tests on Thursday morning."
Nesta limped off as a precautionary measure after a knock during the 4-0 win over Siena on Sunday, but now finds himself a doubt for the big Inter match.