Celta currently sit in the 11th spot in La Liga, but Herrera insists that they are well capable of surprising United.
The first leg of the tie will be played at Balaidos on Thursday evening.
"It's crucial for us," he said via The Express. "Not just because it's a European title, which is important, but also the fact that you can qualify for the Champions League and play in a European Super Cup final.
"Celta are a very complicated team, with players that have a lot of quality. In theory, we are the strongest team left in the competition but we have to respect all of our rivals because otherwise, they can surprise us.
"They're a very dangerous side. They like to have possession and they rely on high pressing. It's a team that makes life difficult for you and they have good quality players like Aspas."