The Bundesliga champions ran riot at the Allianz Arena to give themselves a huge advantage heading into next week's semi-final second leg at Camp Nou, and Roura has said that they are now facing a gargantuan task.
"Miracles are always hard to make real, 4-0 is a huge deficit to make up," the BBC quotes the coach as saying.
"I don't know if anyone has come back from a score like that in the history of the Champions League, but we'll do what we can. This is a big defeat which hurts us a lot."
No team have ever recovered from a four-goal deficit from the first leg to progress in Champions League history.