Son dreaming of making history after 'unacceptable' Spurs season
The north London side are 17th in the Premier League after a dismal campaign but are hoping to end a 17-year trophy drought in Bilbao.
Son played in the 2019 Champions League final defeat by Liverpool and is desperately hoping to end Spurs' long wait against a United side also struggling badly on home soil.
"It would be very special (to win) and it would obviously be a historic moment," Son told a news conference Tuesday.
"I stayed here 10 years, I just want to win a trophy which nobody has done when I've been here in those 10 years -- tomorrow is going to be a massive day for the club."
Ange Postecoglou's Spurs have lost 21 times in the top flight, an unfortunate club record, and are on course for their worst finish since they were promoted back to the Premier League.
"Where we are now is unacceptable, things are not good," continued Son.
"Obviously we can change all the worst season in the league with the trophy.
"Some seasons we've been outstanding... (but) still we were missing something, still we can change that. I hope tomorrow we can change that."
The South Korea international, 32, said Spurs winning the Europa League could be a turning point in the club's history, and more silverware might follow.
"If we win that trophy we can change the mentality, we can change the history, and we can continue to compete for another trophy," he added.
The winners qualify for next season's Champions League, which could mean earning up to £100 million ($134 million) in television revenue and prize money.
Son recently returned from injury and said physically he is "ready" to face Manchester United.
The winger said he had not spoken to departed Tottenham great Harry Kane ahead of the final, but did call him after the England international won the Bundesliga with Bayern Munich earlier in May.
It was the first trophy of Kane's career and Son is hoping the Europa League can be his.
"Everyone knows Harry is my best mate and it was a huge honour playing alongside him," said Son.
"After he won the first title we had a chat about his trophy... He's already on holiday, so he doesn't want me texting him.
"I just concentrate (by) myself, with the boys, Harry will be the biggest Spurs fan tomorrow, I'm definitely sure he will be supporting us from holiday."