The Reds were never in the game and, a week after beating Lower Saxony rivals Wolfsburg to start the season in style, they have returned back down to earth unceremoniously.
"They tore us apart a little," admitted Slomka. "We couldn't find a way of competing with these well-schooled Gladbach forwards.
"Particularly going forwards, we have to do a bit more. This was not a good performance from us."
For Gladbach coach Lucien Favre, the win was the perfect reply to an opening day defeat to Bayern Munich.
"We can be very happy," he told Sky television.
"It was also important because when you lose on the first day of the season, you are immediately under pressure.
"We did not really know after the Bayern game where we stood, but today was better, even though there is still a lot more to do."