"It will be very difficult," he told UEFA.com. "We've got to come back from 4-0 down and that will be very tough, but we'll give it everything and leave with our heads held high and knowing we've done our best.
"For now we have to congratulate our opponents. They played with far greater intensity than we did. We just couldn't score. We didn't fail – we just didn't create enough chances.
"As I said before, they deserved to win. When you come up against an opponent that's better than you, the only thing you can do is congratulate them."
The second leg will be held at Camp Nou in Barcelona next week.