The German champions romped to victory at the Allianz Arena with Thomas Muller's brace plus goals from Arjen Robben and Mario Gomez inflicting the Catalan club's joint-heaviest defeat in European football.
Barcelona were some distance below their best and now need a miracle in the second leg at Camp Nou next week if they are to reach the Champions League final at Wembley in May.
Roura told reporters: "We have to compete in the second leg and try to win. We will give it our all.
"We lacked the freshness to compete with them which is something we have to look at.
"We didn't play that badly in the first half but then they scored early in the second and from there it was difficult.
"We didn't see situations where we could have changed things after the break. We know the situation is difficult.
"Bayern Munich have to be congratulated. They played a great game, they were very strong and they are a very physical team.
"Bayern were the better team and we congratulate them. We had possession in the first half but didn't do much with it."