The Spanish 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 tonight (8.45pm).
“We may be a small club and a team that others don’t take much 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.”
Manchester United come into the game battling a slew of injuries.
Defenders Eric Bailly and Luke Shaw went off in the 1-1 draw with Swansea on Sunday.
But Bailly and backline colleague Chris Smalling and Phil Jones all trained yesterday with coach Jose Mourinho set to risk it all to get into the final and book a place next year’s Champions League should they lift the trophy.
Red Devils winger Ashley Young is desperately hoping that the players will have recovered in time and get back to winning ways.
He said: “It wasn’t [the result] we wanted against Swansea,.
“We’ll have to recover again. It’s not ideal getting a few more injuries on top of the ones we’ve already got but we have to keep going.
“We have plenty of incentive to get ourselves up for the Europa League semifinal and a chance to get to the final. If we can win that, we’ll get into the Champions League.”