Torres says edging a five-goal thriller isn't good enough.
He said: "Shakhtar are a great team. They have showed they have a great mentality. They came back twice and we were lucky at the end scoring the last goal in the last minute of the game.
"I'm happy with the result but we know we must do much better. It's not the way we're used to scoring. We know we have to improve.
"We caused problems for them, pressing and creating chances from their mistakes, but we did not create when we had the ball like we're used to.
"When we have the ball, we know we are dangerous and we can create some chances but it was not the case.
"We were a bit worried about that at half-time and we conceded another early goal in the second half. This kind of situation cannot happen again because, especially in the Champions League, at times like these you can be out."