The former Manchester United forward has scored five goals in the past three games.
Longoria said: "We took the decision internally. OK, there was some opposition, that was objective, but at the same time that gave us power to maybe not investigate, because I am not a judge, but to use all the information to take the best decision, which I think we did."
He added: "In our very first conversation with the coach (Roberto de Zerbi) we mentioned Mason's name.
"The coach knew Mason from the past and it was a very quick conversation at the start between all of us because we believe in the talent of Mason.
"There were a lot of conversations between all of us in the weeks leading up to his move. He is behaving fantastically with us and we are very happy to have him playing in our team."
On England refusing to consider Greenwood for selection after assault charges were dropped against him last year, Longoria said: "I don't want to answer that kind of question out of respect to everybody.
"I prefer to talk about what Mason is doing in France and Marseille and we are very happy with the kind of performances that he is doing during the first weeks as well."