How does Willy defend Biff

Willy would like to see himself as a successful version of Biff. In defending Biff, Willy is defending an idealized version of himself. Biff is a football player; he is boyish and looks like an all American success story. Willy makes excuses for the things Biff is not successful at that Willy wanted for himself. He defends Biff over things like his failed football career and inability to find meaningful work.