The 24-year-old, who has partnered Mats Hummels at the heart of the BVB defence since 2009, believes his side conceded too many goals last term and fears they will come up short if they fail to eradicate the problem.
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"This figure has sharpened the senses and made us realise we must defend better. This will not happen again, and we have done well in pre-season."
When Jurgen Klopp's team won consecutive Bundesliga titles in 2011 and 2012 they conceded just 22 and 25 goals respectively, and Subotic says that a dip in form was, perhaps, inevitable.
"Maybe this was always going to happen. We are only human," he continued.
"We got complacent and now we must focus again. I am confident that if we can stop the small errors it will be a successful season.
Subotic joined Dortmund from Mainz in the summer of 2008 and has gone on to make 188 appearances for the club, scoring on 15 occasions.