The Galician club have never won a major European trophy while United have lifted the European Cup three times.
"I think with the mentality we've built here, nothing is impossible,” Swede Guidetti, a former Manchester City player, told UEFA ahead of the first leg in Spain on Thursday.
“We may be a small club and a team that others don't takemuch notice of, but we also know how good we are.
“That's why we've beaten Barcelona two years in a row. We knocked Real Madrid out of the Copa (King’s Cup), we've beaten Atletico Madrid. We've shown that we are a good team.” Celta Vigo, 11th in La Liga, lost 3-0 to Athletic Bilbao on Sunday but coach Eduardo Berizzo rested several players including leading striker Iago Aspas.
Manchester United come into the game battling aslew of injuries.
Defenders Eric Bailly and Luke Shaw went off in the1-1 draw with Swansea on Sunday, joining an injury list that also includes Zlatan Ibrahimovic, Chris Smalling, Phil Jones, Marcos Rojo and Timothy Fosu-Mensah.
France midfielder Paul Pogba is fit to return after injury.
Fifth in the English top flight, United are at risk of missing out on a Champions League place for next season and their only hope of playing in the competition might be winning the Europa League, a trophy they have never lifted before.
But Spaniard Mata warned his team mates that seeing off Berizzo’s team will not be easy. “They are one of the teams that play better football in Spain, and not only this season,” Mata said.
“Being back home in Spain will be special for me but difficult at the same time. We must perform at our best in order to beat them.”
United are looking to join an elite group of clubs who have won the Champions League (formerly European Cup), the European Cup Winners’ Cup, and the Europa League (formerly UEFA Cup).
Ajax, who face Olympique Lyonnais in the other semi-final, Chelsea, Bayern Munich and Juventus are the other sides who have achieved the feat.
(Editing by Ed Osmond)
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