The match finished 1-1 before Almagro won the penalty shoot-out 3-1.
De Rossi, who spent his entire career in the Italian capital, told fans before the game he had come to Argentina not for the money, although he will doubtlessly be well paid, but for the romance of turning out for one of the world's most storied teams.
"I've been passionate about this club since I was little," the 36-year old said. "This club allows me to play at an excellent level and in the way I enjoy."
De Rossi's arrival is unusual in that he has no links to the Buenos Aires club, other than a friendship with their sporting director, his former Roma team-mate Nicolas Burdisso.