A youthful and inexperienced Tottenham side lost 1-0 at Pacos de Ferreira in the first leg of their Europa Conference League play-off tie on Thursday.
The visitors made 11 changes from the team that beat Man City in their Premier League opener on Sunday, with head coach Nuno Espirito Santo handing full debuts to new signings Pierluigi Gollini, Cristian Romero and Bryan Gil.
However, it was Lucas Silva's well-taken goal on the stroke of half-time that proved the difference between the two teams heading into the return leg in north London next Thursday.
- How the teams lined up | Match stats
- Tottenham interested in Chelsea defender Kurt Zouma
- Harry Kane trains with Spurs team-mates for first time this season
How Nuno suffered his first loss as Spurs boss
Spurs headed to Portugal having left behind every player who featured against the Premier League champions on the weekend, allowing Nuno to get a first glimpse of summer signings Gollini, Romero and Gil.
It was the latter - who arrived from Sevilla in July - who initially caught the eye, with one fine early run down the left almost creating an opener for Ryan Sessegnon, making his first Spurs appearance since March 2020 having spent last season on loan at Hoffenheim.
Gollini also impressed in goal with some confident handling in the first half as the hosts, who finished fifth in the Portuguese league last season, gradually grew into the contest in Porto, before deservedly taking the lead just before the break.
Tottenham handed a full debut to 17-year-old Dane Scarlett as Harry Kane and Son Heung-min were left at home, with Nuno Espirito Santo making 11 changes from the side that beat Manchester City.
There were full debuts for new signings Pierluigi Gollini, Cristian Romero and Bryan Gil, while 18-year-old Nile John was also been handed a full debut. And Ryan Sessegnon made his first Spurs appearance since March 2020.
It was the third new signing, though, who was partly at fault as Romero and his fellow centre-backs were split open by Castro Ferreira's through ball, allowing Lucas Silva a clear run on goal.
The striker kept his cool to slide his low shot past the advancing Gollini as the home team led at the interval, with the visitors struggling thereafter to get back into the contest.
In fact, a lacklustre Tottenham were unable to even force a save out of Andre Ferreira in the second period, having no shots on target in the entire match, and now have it all to do in the return clash in north London next Thursday.
What the manager said...
Tottenham head coach Nuno Espirito Santo: "Not good, not good. First half we lost too many passes - and the goal came from that - we were imbalanced. But not a good performance, honestly not a good performance.
"Nobody likes to lose a game or not play good. The feel-good factor disappears. This is football. It is up and down but now you realise that this is the situation, they have to bounce back. This is what we are going to do."
Nuno on Kane: "He trained today. He trained with the group of players that stayed in England. He trained well and he is going to train Friday. And when we return he will join all the players and work with the group."
Pacos host Estoril Praia in the Portuguese top flight on Monday at 7pm.
Meanwhile, Spurs travel to Wolves in the Premier League on Sunday, with the game kicking off at 2pm, before they host the Portuguese side in the return leg next Thursday night (7.45pm).