Peter Stoger admitted Borussia Dortmund's nightmare display against Red Bull Salzburg left him struggling to sleep.
The Bundesliga club slipped to a 2-1 home defeat in the first leg of their last-16 Europa League tie on Thursday, leaving them with a mountain to climb when they travel to Austria for the return fixture next week.
Valon Berisha scored twice for the visitors and while Andre Schurrle bundled in a goal to halve the deficit, Dortmund slipped to their first defeat in 2018.
With little time to reflect before facing Eintracht Frankfurt in domestic action, Stoger revealed he had sought the opinions of his staff to try and work out why Dortmund performed so badly.
"I didn't sleep very well, and not for that long either," he told the media.
"It doesn't happen all the time, but sometimes you're reviewing the matches immediately. You're talking with some guys about the impressions you got from the match.
"Clearly there isn't very much time until the next game, so you want to hear their point of view. Alternatively, you have a short and uncomfortable night."
Frankfurt – who sit in fourth place in the table – fought back from 2-0 down to draw with Peter Bosz's Dortmund last October.
Stoger knows the visitors to Signal Iduna Park will make their presence felt and warned his players to be prepared for a physical battle.
"The team has to show a lot of character," he added.
"Eintracht are a team who play physically, they are aggressive, well organised and combative.
"Normally we are strong creatively, using our speed on offence. That will still be necessary, but it won't be enough. We have to fight in the duels and take our chances."
There was some good news for Dortmund in the aftermath of the Red Bull Salzburg defeat though, with the club announcing Marco Reus has signed a new long-term contract.