Juve fell 2-0 to Lecce on Sunday.
"The group is always united, but we are not compact when we go out on the pitch against the little teams," Marchisio said.
"We don't have the mentality of a great team. We must improve. The team is angry, very angry.
"Del Neri is very angry, but we know among ourselves that we played very badly. Right now I am really not thinking about fourth place.
"The only thing that we can do is remember the horrible feelings that we feel now.
"We aren't afraid. We don't lack desire. We must be more ruthless."