Brendan Rodgers insists Celtic still retain hope of progressing in the Europa League despite a 2-0 defeat by Valencia at Parkhead.
Russia international Denis Cheryshev scored for the visitors three minutes before the interval in the first leg of the last-32 encounter.
And the second came in the 49th minute when debutant striker Ruben Sobrino knocked in a cross from Cheryshev, with the Spanish side running out comfortable winners ahead of the second leg next week at the Mestalla Stadium.
The Northern Irishman said: "Valencia are a team who wait on mistakes, very organised, compact and we made too many mistakes especially in the first half under no pressure.
"We gave it away and when you do that against good teams it kills your momentum and it puts you on the back foot. The lessons are you need to really concentrate.
"It is so important. You can get away with it domestically sometimes but certainly at this level against good players, if you bring pressure on to yourselves it makes it difficult for us.
"If you make the mistakes at this level you get punished, the disappointment is that it was basic mistakes and they capitalised on the two mistakes.
"We have to try to learn from it. We have a lot of young players, we have to take that on board and take it on the chin.
"We go next week to Valencia, it will be a very difficult game for us but if we get the first goal then it can bring a different possibility for us.
"We have to have that belief. We have got good players and we might have one or two back for then but we need to defend and concentrate well in key moments and, like I say, if we get the first goal then it could set us up for a really good second leg."
Valencia boss Marcelino, speaking through an interpreter, was impressed by the "extraordinary, incredible" Parkhead atmosphere and pleased with his side's performance which makes them firm favourites to reach the next round.
He said: "It could have seemed easy but it was difficult. It was a good match and we played well, so respect to the players.
"We didn't lose too much of the ball and we defended and attacked very well. I am very happy and satisfied with the way things went.
"This tie is not decided yet but it would take a huge error not to qualify from this."
