Choco Lozano emerged to hit the winner for the visitors.
Courtois said, "We'll have to analyse the match and take a look at things. Our first-half performance was very poor, I was called into action three or four times, we weren't winning our inidividual battles and they were better than us in virtually every department. The gaffer made changes at the break and we were better in the second half, but we failed to score because they're a very well-organised side who defend very well. It's really tough when you go behind to teams like that.
"We'll have to talk about it tomorrow between ourselves. Besides yesterday's session, all 20 of us haven't trained together and with the international matches it's difficult to get everyone together to train, but that's no excuse."
He continued: "A defeat is always a good thing because it gives you a wake-up call to get back down to work, realise the things we're doing badly and to improve. We shouldn't attach too much importance to it. It's a defeat, in particular you've got to win your home games, but the games now come thick and fast and we'll have to put in a good performance against Shakhtar, win the match and head to the Camp Nou in confident mood."