United face the Spanish side in the second leg of their Europa League last-32 tie this evening with a 4-0 lead after their impressive performance last week.
Imanol said: "It's the best Man United in the last 10 years.
"They're battling with Man City for the Premiership. They're really world-class physically, and quality-wise, they're really good.
"We know how difficult it is to cause nerves in Man United even if we score that first goal. But we want to be competitive.
"We're going to try and go out there to win the game but, above all, we need to prove ourselves that we are not that far (from United).
"We might be far from Man United but we're not as far as the result of the first leg suggests."