Celtic coach Brendan Rodgers admitted his side got “spooked” in Tuesday’s 7-1 Champions League loss at Borussia Dortmund.
Dortmund were 5-1 up at half-time, with winger Karim Adeyemi scoring a hat-trick and winning a penalty before the break.
Emre Can gave Dortmund a seventh-minute lead from a penalty. Serhou Guirassyalso converted from the spot and scored a solo goal. Felix Nmecha added another as Dortmund equalled their best ever Champions League performance.
Celtic’s Daizen Maeda had equalised at 1-1 after nine minutes, but Dortmund retook the lead through Adeyemi just two minutes later.
Rodgers said the match was “a tough watch.”
“We weren’t at our best and they showed why they’re a top, top team.”
“We might have got spooked a little bit early. At this level you need to be at your highest level and we weren’t — and they were.”
Celtic had won eight from eight to start the season but finished on the wrong end of a dominant performance from last year’s Champions League finalists.
“We went in high in confidence,” said Rodgers. “The team has been playing ever so well. We were in a really good place but we needed to start the game better than we did.
“We got punished. It was absolutely ruthless in the finishing. It was incredible to see unfortunately for us we were on the back of it.”
Celtic’s next Champions League match is away at Serie A side Atalanta, who won last year’s Europa League.
Rodgers said it would be “very difficult for us to get to (Dortmund’s) level, but what we want is to be more competitive.
“What we saw tonight was the very highest level of the European game.”
Celtic captain Callum McGregor told TNT Sports “it’s a sore one.”
“We were punished for every mistake we made, which doesn’t happen often in a game of football.
“Before we knew it, we were 5-1 down. A sobering night.
“We have to stick together. We’ve had a good start to the season but we’ve been sobered a bit.”