Lewandowski scored both goals as Barca won 2-1 at the Mestalla.
"In the first 20 or 25 minutes we didn't play very well, but then we improved. In the first half we had problems reaching the rival area, but finally the goal came," said Lewandowski.
"In the second, we have been much better, with three or four very clear chances. This match is important because it marks a good start to the season. Valencia is always a difficult opponent and we have taken the three points."
The Pole also praised the performance of the young players who featured, includinf Marc Bernal, Gerard Martín (who made his debut) and Marc Casadó.
Lewandowski added, "We can win games with young players Normally in this type of field you need to have experience, but the young players have played very well.
"They have great potential."