Porto announced the surprise signing of Luuk de Jong before going on to beat Atletico Madrid 1-0 in a pre-season friendly.
De Jong was unveiled on the pitch ahead of kick-off on Sunday.
The 34-year-old has joined Porto on a free transfer, having left PSV following the expiration of his contract.
De Jong scored 14 Eredivisie goals last term, while supplying eight assists. He converted 14.3% of his shots, underperforming his league-leading 18.0 expected goals (xG).
The Dutchman then watched on as his new team overcame Atleti thanks to Victor Froholdt's goal on the stroke of half-time.
Diego Simeone's side have been busy in the transfer market, though only two of their new additions – Matteo Ruggeri and Alex Baena – started.
Atleti, who failed to progress to the Club World Cup knockout stage, only have one more pre-season friendly.
They take on Newcastle United next week before they kick off their LaLiga campaign against Espanyol on August 17.
Elsewhere on Sunday, Serie A champions Napoli went down 2-1 to Brest.
Ludovic Ajorque netted twice for the French side, with Lorenzo Lucca on target for Napoli.
