Cassano has spent the last two days in hospital after being taken ill following the 3-2 win over Roma.
Reports from Italian news agency Ansa claimed he had suffered a stroke.
“We’ll be playing for Antonio. Quite apart from the sporting element, what counts most is the human aspect and we’re all feeling sad.” said Allegri.
Cassano had complained of finding it difficult to speak or move when the team arrived back in Milan in the early hours of Sunday morning. He has since been undergoing tests on his heart and brain in hospital.
Allegri added: “Antonio was doing well and he was in good shape and I hope he’ll be back as soon as possible. “The doctors are working on it, let’s hope it all goes well. I can’t diagnose him but he sounds fine. It was completely unexpected and has nothing to do with football. Let’s leave it to the doctors and I think everything will be fine.”