Liverpool raided Porto for Luis Diaz in January and were back watching the Portuguese side on another scouting mission this week.
The Reds sent scouts to the Estadio do Pragao for Wednesday night's Europa League clash.
Porto were undone by Lyon, who now hold an advantage ahead of the return leg, which meant few of their current stars were able to impress.
The Portugal outfit have long been an impressive breeding ground for players, especially those from South America.
Clubs have captured the likes of Radamel Falcao and James Rodriguez from Porto and Liverpool are exercising the possibility of doing so again.
Manchester United also had representatives in the stands as they kept their eye on any possible targets whilst German giants Bayern Munich also had onlookers present, According to newspaper O Jogo.
Midfielder Vitinha was a bright spark on the night as the hosts were beaten. Porto currently sit top of the league with Mehdi Taremi their top goalscorer following Diaz's departure.
Liverpool have been able to impressively transition their forward line in the past 18 months.
Previously Mo Salah, Sadio Mane and Roberto Firmino looked undroppable and questions were asked as to how Jurgen Klopp would rebuild once any of them began to drop off.
Diogo Jota came in last term and hit the ground running, eventually replacing Firmino.
Now Diaz has arrived and had an instant impact which has allowed Klopp to rotate his frontline, now utilising Salah centrally to accommodate his January signing.
The German has been full of praise for the Colombian, who has immediately adapted to the demands of English football.
"Luis is a top boy, a top class player," the Reds boss said earlier this week. "He showed that already in two appearances he had for us.
"Last game he didn’t come on. He is a new player, we have to get used to new things in a team, we have all other players fit and they know exactly what I want from them.
"We give Luis a little bit more time to adapt to it. He is an incredible player, his joy and love for football is so obvious.
"When you see him training, he cannot stop smiling. I have never had a player like this. It's crazy. We are so, so happy to have him here. It's absolutely great.
"We expect a lot from him in the future. Now already but in the future as well. I have a good feeling it will be a really nice story with Luis at Liverpool."
