The Eredivisie side had to come from behind twice in the UEFA Europa League Group E clash in Athens before Wijnaldum scored the decisive goal in a 3-2 win.
Trailing 2-1 at the break, the 23-year-old explained heated discussions inspired PSV's second-half comeback, his winning goal coming 12 minutes from time.
"We played a good game, but we should have never given away two goals in the first half," he said.
"In the break we became very angry. The sharpness was not there.
"The coach was angry, but we as players were angry as well
"The way we then responded told a lot about the growth this team has taken together. We are a year older, we have more experience and everyone is working together."
PSV sit second in the group after four matches, five points behind leaders Dinamo Moscow, and will qualify for the knockout stages by avoiding defeat against Estoril next time out.
Domestically, Phillip Cocu's side have impressed with nine victories in their 11 matches to top the table, four points clear of rivals Ajax.
