Three minutes into stoppage time Noah Sadiki hurdled a set of advertising hoardings sprinted across a running track and hurled himself at the jubilant Sunderland fans.

Just as those supporters' long awaited party in Portugal had looked in peril of falling a little flat, the Democratic Republic of Congo midfielder capitalised on excellent work from Enzo Le Fée and Granit Xhaka to lash home an eye-catching first goal for Régis Le Bris's side.

It meant Sunderland had come from behind to secure a second straight Europa League win and their relief was evident as every outfield player followed Sadiki in vaulting those hoardings to join the celebrations.

With Torreense's home ground an hour down the motorway towards Lisbon in Torres Vedras, too small to accommodate European football, Leiria's stadium stood in.

It proved quite some substitute. An arena overlooked by a 12th-century castle constructed to defend this attractive medieval town in central Portugal against reconquest by the Moors created an imposing backdrop to Sunderland's first competitive trip to the continent for 53 years.

Back in 1973 they were second division FA Cup holders. Torreense know the feeling. Having provoked widespread shock by beating Sporting to win the Portuguese Cup last spring Luís Tralhão's second tier side relish their underdog status.

After winning at Lillestrøm last month, Torreense aim to keep the surprises coming and duly left a heavily rotated Sunderland trailing at the break.

Le Bris had left Xhaka, Le Fée and Nordi Mukiele on the bench ahead of Sunday's vital Premier League trip to Bournemouth.

This reshuffle permitted Luke O'Nien, a survivor from League One days, to captain Sunderland from central defence and he headed an early chance from a corner narrowly wide.

There were plenty of travelling supporters to cheer O'Nien and co on as the ground turned into a sea of red and white. Although only 4,000 official tickets had been given to Sunderland they were readily available locally and estimates suggested the formal allocation had doubled.

As the sun dropped and the temperature fell from a high of 28C those fans watched a rather unconvincing first half on Sunderland's part. It seemed telling that the normally fairly impassive Le Bris looked distinctly, and unusually, irritated and frustrated at times. Granted his humour was hardly helped by the referee's penchant for detecting phantom Sunderland fouls but ominously Malick Fofana, a £30m summer signing from Lyon, was struggling wide on the left. Ditto Nilson Angulo on the right wing.

Torreense, 15th in Portugal's second tier, might not have been threatening Robin Roefs's goal but they were succeeding in blocking and clearing pretty much everything that came their way. Indeed Tralhāo's defence looked almost as well fortified as that ancient castle looking down on them.

Admittedly, midway through the first half, Timur Tutierov, Sunderland's centre forward for the night, thought he had headed the opener but that effort was disallowed due to a spot of shirt pulling on Omar Alderete's part.

Then, with the interval, fast approaching Le Bris adopted hand on hips mode as Torreense reminded everyone precisely why they have built such a giantkilling reputation by scoring with their first shot.

It came from Volnei Feltes. With Sunderland losing concentration and standing off, he was permitted to advanced unopposed before cutting in on his right foot and sending a 20 yard curler curving, sublimely, into the top corner.

Although Angulo forced Lucas Paes into a fine headed save he, like Fofana, was soon replaced as Le Bris made a quadruple substitution involving the introduction of Le Fée, Xhaka, Mukiele and Reinildo.

This infusion of quality left Torreense a little groggy and the midfielder Jules Ahoka – a 20-year-old £8.5m summer buy from Royal Antwerp – was able to equalise by heading his first goal for Sunderland courtesy of Mukiele's deft dink.

Xhaka then conjured a wonderful opening for Ahoka but he could only aim straight at Paes, a draw beckoned. Then Tutierov was brought down in the box and the referee immediately pointed to the spot. However, a linesman's flag indicated an offside so it seemed there would be no penalty. Or at least not until VAR intervened – lengthily – and it appeared the eventual conclusion was there had been no offside and no foul.

No matter, when Le Fée dinked a gorgeous ball into the box and Xhaka crossed, Sadiki waited for it to drop before applying the finishing touch.