Carles Aleña played 90 minutes on Thursday as Barcelona were beaten 2-1 by Levante in the Copa.
"It's important to start well and be focused from the beginning in these type of games, but we went out asleep and we paid quickly for that," the midfielder said.
Aleña added that Barça "didn't play well, but there are 90 minutes left in the tie and, without doubt, the game [at Camp Nou] will be long for Levante."
Ernesto Valverde made a number of changes for the game with Lionel Messi, Luis Suarez, Ivan Rakitic, Gerard Pique and Jordi Alba all sitting it out.
But Aleña says it's not right to blame the loss on rotations, with B team defenders Miranda and Chumi both subbed off.
"You can tell we've not played games together, but we train every day and we know each other," he said. "It's not an excuse. The reality is we didn't play well. ortations aren't an excuse."
Finally, he was asked about Neymar's possible Barça return: "I don't know. We don't talk about it in the dressing room. He's a special player who would improve any team, but we are completely focused on this season.
"I don't know if those that know him more speak with him and if he wants to return. We will see what happens."