La Real are at Old Trafford tonight for the second-leg of their Europa League round of 32 tie. United hold a 4-0 first-leg lead.
Imanol says the team could feel the difference in level.
"In Turin we saw that we still have a very long way up to United's level. However, the difference may not be as great as we felt right after the match," he said.
"I have no doubt that they will go on the field again to win again - maybe even as big as last time. This is how it is to be a great team."