The Signal Iduna side were without their goal-getter Erling Braut Haaland and captain Marco Reus with the duo being replaced by Donyell Malen and Youssoufa Moukoko.
And Hummels who is of the view that the sending off of midfielder Mahmoud Dahoud made it difficult for the side to find an equaliser aside the absence of Haaland and Reus.
"We made our own lives very difficult with the sending-off," said Mats Hummels after the final whistle of the game away to Borussia Mönchengladbach.
"The naturally match became extremely difficult as a result." "we were missing two central elements of our game in Erling and Marco," explained the 32-year-old center-back, who won 70% of his duels today and was by far the strongest-performing Dortmund player in the challenges.
Hummels added: "We were also missing other players in attacking positions. We lacked penetration at times. Nonetheless, we could've done better. It was a 50-50 game until the sending-off. We worked quite well defensively and showed willpower in the second half, but we didn't deliver in the decisive moments."
Dortmund will return to action on Tuesday when they host Portuguese giants Sporting in the Champions League.
Dortmund are currently the table-toppers in Champions League Group C after winning their first game against Turkish Super Lig team Besiktas.
