Before Manchester United arrived in Galicia six days ago, the last time Celta Vigo had faced an English team Iago Aspas was a ballboy. He was there in his usual place pitchside at Balaídos wearing an oversized grey bib when Thierry Henry and Robert Pires played their way past Eduardo Berizzo to score an 80th-minute winner; a fortnight later, he watched on television at home in Moaña, across the water from Vigo, as they were eliminated at the last-16 stage at Highbury. It was the first time they had been in the Champions League and they have not been back.

At the end of that season, they were relegated. Tonight, 13 years on, they return to England. Manchester awaits; if Celta can overcome a 1-0 deficit, so does a first European final. “We’ve been thinking about this all week,” Aspas says. He has been thinking about it longer than that. “It is difficult,” he says, “but it is a dream.”

Much has happened since then. The day before the second leg in London, José Mourinho, a young manager with a solitary league title to his name, took a step closer to his first European Cup and sprinted up the Old Trafford touchline that is now his own. Berizzo, the centre-back beaten by Pires and Henry, has become Celta’s manager. And as for the ballboy, he has become their everything. Well, almost. Aspas says he is “just another player” – but he is not. Even Hugo Mallo, the captain leading them out on what most consider the biggest night in their history, joined the Iago Aspas Supporters’ Club.

The first time Aspas tried to join Celta he was turned away when they discovered he was not old enough, but he did sign up aged nine. Twice he left – he briefly walked out in a huff at youth level and then in 2013 he signed for Liverpool before going from there to Sevilla – but both times came back. The spells away helped him re-evaluate and his return helped him refind himself. From their Madroa training ground, 500m above the Ría de Vigo, where the mist rolls across the pitches, he can make out Moaña on the other side, 20 minutes away over the Rande bridge in his worryingly noisy old white VW Golf. “Home,” he calls it, “where people love me.” Laughter booms out from the dressing room after their final session before flying north.

At Liverpool. Aspas took a corner, which has followed him since, even a superb international debut goal at Wembley insufficient to eclipse it entirely. But if you remember him like that, Celta supporters remember him like this: as the 21-year-old who came on as a late substitute and scored twice on his debut, including the injury-time winner that rescued Celta from relegation to the regionalised third tier in 2009 and, with it, possible extinction; as the striker who led them to promotion in 2012 with 23 goals; and as the man who rescued them from relegation in 2013. The “4%”, they called it – Celta’s mathematical chances of survival before he scored in the penultimate game and gave the decisive assist the week after.

Mourinho has done that at almost all his teams. There are times when the referee accepts it, times he doesn’t

Aspas has been at the heart of all their most significant recent moments, all the way to this – the most significant of them all. Even his departure was good for them: when Liverpool came, Celta desperately needed the money. It might have been good for him, too: Aspas was “all heart” says one former coach, occasionally losing his head, like the time he was sent off in the Galician derby, and some locally talk about a cura de humildad, a dose of humility and maturity. Returning was a revival, another chance back where he belongs. At 29 now, he is their leader.

At the end of the corridor where the dressing rooms are is a door, on to the first team training pitch. “Earn it,” it reads in galego. Aspas has done. In 2016, he scored the two goals that, with Valencia drawing the next day, took Celta to Europe for the first time in a decade. He scored to take Celta to a European quarter-final for the first time in 16 years. And he scored again to take them to a ever European semi-final.

“No one bet a duro on us getting here,” he says. A duro is a five-peseta coin. Being here is a huge achievement, they know, and facing United is a huge occasion. Perhaps they know it too well, as is implied by the half-and-half scarves handed out at Balaídos. “We respected them too much, especially in the first half,” Aspas admits. Celta were not Celta. Nor was he himself. “I had someone on top of me all the time,” he says. “I got space when I came into the middle in the second half.”

Celta Vigo’s Iago Aspas battles for the ball with Marouane Fellaini of Manchester United in the Europa League semi-final first leg. Photograph: Juan Manuel Serrano Arce/Getty Images

United were not a surprise, Aspas says. The difference between the clubs is an inescapable reality – Juan Mata, a United substitute, cost four times as much as Celta’s entire squad, while this was United’s 16th European semi-final – and the physical difference in the middle was not entirely unexpected, overrun though Celta were.

Nor, he says, was the way the final minutes were played – or not played. John Guidetti complained that of the final half-hour, barely three minutes were played. But Aspas says, simply: “Mourinho has done that at almost all his teams. There are times when the referee accepts it, times he doesn’t. It’s part of football. We’re ready for that – we don’t have much choice. If they’re leading, we’ll have to be. And if we’re leading, I suppose we’ll try to do the same.”

“Paul Pogba is big but we knew that; we know English teams are very physical, fighting for the second ball. It’s very different from the Spanish style. We were much better in the second half but they got in front, which let them sit back and made it difficult to hurt them. Now we have another chance. We reached the semi-final of the Copa del Rey. We were safe with two, three months to spare. And we’ve reached this semi-final. We have one more bullet, which we have to use well.”

It won’t be easy. Jodido is the word Aspas uses: literally screwed, it translates better as very, very hard. But that does not mean giving up; it means being braver, more like Celta, the way Aspas has always played. A team-mate from his debut jokes: where are you going with that “tin chest” of yours? He does indeed look almost like he is wearing a breastplate, caving in at the sternum. Watch him and there’s something in that, an illustration of his game: small, slight but straight at them, chest out. Revolucionado, is the word. “I don’t like losing,” he says.

“We owe the fans,” Aspas continues. “We didn’t repay all that support. As soon as the first game ended, you could see everyone thinking it over. We know it’s hard, who we’re playing, what the situation is, but …”

So how then? “If I tell you that, it will be in the paper tomorrow and Mourinho will know,” he replies. “But we have to play our way, with no fear; the only thing that’s any good is a win, so we have to go for it. We have to score and the sooner the better. We have to be patient, though – not go mad trying to turn it round in 20 minutes. We’ve stood out as a team that scores lot of goals and plays well away. We hope to keep that up.”

Iago Aspas was not given many opportunities by the then manager Brendan Rodgers when at Liverpool. Photograph: John Powell/Liverpool FC via Getty Images

Celta have never been to Old Trafford but Aspas has won there, 3-0 in 2013-14. The other time he went, he was left out as Liverpool were knocked out of the Cup, but that first day he got a handful of minutes – among the very few he played in a disappointing Premier League season. He lived near Sefton Park, his girlfriend and brother joining him, a group of mates coming along for a while, too, but he did not take much away with him: a solitary goal and not many more English words, although the way he says “fuck off” does at least sound a little scouse. Aspas says: “Scoring the winning goal there would be a dream come true and I know Liverpool fans want us to win.”

He still talks to Raheem Sterling and Luis Suárez among others and speaks fondly of Steven Gerrard, a man with “presence”, a captain who “didn’t say a great deal but when he did everyone listened”, and suggests that Spain could learn from England when it comes to the “respect” for players such as him. He also insists that the experience was a good one, despite the lack of minutes, light or language. “I’d get up, go training, eat, have a siesta and when you wake up it’s night,” he says, smiling. “I did try but we almost always spoke Spanish. There were seven or eight of them that spoke Spanish and so it was inevitable.”

The number rises. The next time he mentions language it’s nine. Then 10. Then there’s Glen Johnson, who spoke Spanish, too. Oh, and Brendan Rodgers gave him instructions in Spanish.

What Rodgers did not do was give him many opportunities. “It was their best year for 10, 15 years, they were close to winning the league. [Daniel] Sturridge scored 21, Luis [Suárez] 31, so it’s normal that I didn’t get many chances. It didn’t annoy me; I understood that I couldn’t play with them at that level. It’s hard, of course: I’d played every game at Celta. But there are times you can’t play and when they win [without you], you know that. You have to take your frustration out on the training ground and the sessions were really good, at least.”

His corner taking, on the other hand, was not. Mention Iago Aspas and the inevitable response recalls the final minutes of that decisive meeting with Chelsea at the end of that 2013-14 season, when, with time running out and the title slipping away after Steven Gerrard’s slip and Demba Ba’s goal, Aspas sent a corner to the edge of the area straight to Willian. That error was recalled again after the first leg, as if some sort of definitive proof against him. It does not matter that he took Celta there or that only Messi, Suárez and Ronaldo have scored more in La Liga this season, or that he went to Wembley and scored, it’s all about the corner – a portrait of failure.

Aspas says: “I don’t have to prove anything to anyone; I just have to help my team. I have to be the player I’ve been this season. I already proved myself when I went there with the national team.”

Everyone else has gone; he is the last man left at Madroa apart from the member of staff huddled in a cramped store cupboard under the stand by the door, surreptitiously puffing away on a cigarette. Aspas gets up, slaps him on the back, swears at him and climbs behind the wheel of his car, noisily heading the short distance over the bridge and home. But not without a final word on that corner.

“I suppose it’s because it was my last game and we lost,” he says. “It was a great season for Liverpool – one of their best – although I would have liked to have played a bigger part and that corner is the final memory, so it’s their lasting image of me. But in football you can’t forever live in the past. Things keep moving, every day gives you another chance. Hopefully, for the sake of Celta and because of the happiness it would bring Liverpool fans, we can knock out United and they can have a new image to replace it.”