The defeat was Bayern Munich's heaviest in the history of the German cup.
The Bavarians paraded all their top stars including the likes of Robert Lewandowski, Manuel Neuer, Joshua Kimmich, Serge Gnabry, Leroy Sane and Leon Gotetzka but still went behind by three goals in the opening 21 minutes at the Borussia-Park.
Bayern Munich's night went from bad to worse in the second half as Breel Embolo added two more to cap a miserable day in the office for the German champions.
Like many others, Salihamidzic was stunned with the events that unfolded before his eyes and was at a loss to explain what exactly happened.
"I'm absolutely shocked," Bayern sporting director Hasan Salihamidzic told ARD after the game. "We just weren't in that match. In every situation they were tougher than us.
"A collective blackout. That was inexplicable. We know how difficult it is always in Gladbach, we discussed that beforehand. It's all difficult to explain, I have to be honest."
The visitors were without manager Julian Nagelsmann, who is still recovering after contracting Coronavirus.
"Of course we want the coach to be there. But the defeat has little to do with Dino, rather that we weren't there for the duels at all.
"Everything that could go wrong really went wrong. It has little to do with the coach today."
Bayern Munich have now failed to progress beyond the last 32 stage of the DFB-Pokal for the second season running.
