Hosts Bayern hit Barca for four at the Allianz Arena to put themselves in pole position to secure a place in next month's Wembley final.
"Bayern have to be congratulated," said Roura. "They were very strong; they are a very physical team. We didn't play all that badly in the first half but then they scored quite early in the second from a strategic move and from there it was difficult.
"Bayern played a great game while we were lacking that freshness to compete with them and that is something that we have to look at. We didn't see situations where we could have changed things after the break.
"We know that the situation is difficult but as Barcelona we have to compete in the second leg and try to win. We will give it our all."
The return leg will be played at Camp Nou next Wednesday evening, with Borussia Dortmund and Real Madrid facing off in the other semi-final first leg tomorrow.