Froholdt suffered a heavy head collision with Moreirense's Guilherme Liberato two weeks ago.
He collapsed but regained consciousness on the pitch, was left on a stretcher, and spent the night under observation at the hospital. Scans showed no abnormalities, and he was quickly discharged but did not feature in Porto's clash with Manchester City on 8 September.
Froholdt made a strong return to the starting lineup on Sunday. He scored the decisive goal to make it 2–1 in the 54th minute—just three minutes after his international teammate Alexander Bah had equalized for Benfica.
After the win, Froholdt said: "It was a special night to play this kind of game against our rivals; it was an amazing atmosphere to be a part of, and I am really proud of what the team achieved today, especially in the second half after we changed the momentum when they scored their goal."
"My head feels fine. I am ready to face Portugal, and I am looking forward to playing for the Danish national team again".
Denmark will face Portugal in Copenhagen on Thursday 1 October. Before that, they will face Norway in Oslo on Thursday 24 September and Wales in Copenhagen on 27 September in the Nations League.