The goalkeeper, on loan from Chelsea, was full of praise for the Catalan club – and Lionel Messi in particular – in the wake of his side's 5-0 loss to the European champions
The Vicente Calderon club were hopeful of achieving a positive result against the Catalans, but they were trailing 2-0 within 15 minutes and were eventually heavily beaten.
"Barcelona play football from another planet. I feared for a real thrashing after their good opening, and, to be honest, it was a bit of a whipping in the end," Courtois was quoted as saying by Sporza.
"We started the match too timidly and paid them too much respect. We should have shown a bit more flair."
Courtois also had high praise for Saturday's hat-trick hero Lionel Messi.
He added: "It's quite a special feeling to see him dribble in real life. He definitely is the best player in the world."
Atletico have dropped to eighth spot in the Primera Division following their loss, while Barcelona now sit second in the table.