A first-half Lionel Messi hat-trick had the Catalans three goals to the good by the 41st minute at the Mestalla on Sunday.
However, Valencia striker Helder Postiga struck a three-minute brace in the shadows of half-time to give the hosts hope of a stirring comeback.
"That sometimes happens in football ... one bad minute at the end of the first half. This is a difficult away game," Martino said.
"This has been the best Barca this season, because of the stadium where we played, the opponent we faced, and the way we played."
"What worries me most is that we gave Valencia the chance to get back into the game, we relaxed when we were three goals up," he said.
"Yesterday there were doubts about the Messi-Neymar connection after their first game together. Things have been solved in 24 hours," the coach said.
"Mascherano got a serious knock on his ear. Busquets could be worse, thigh muscle tear," he said.
