The Bundesliga leaders slipped to a shock 2-1 defeat away to Augsburg yesterday.
While Dortmund still sit three points clear of Bayern Munich in first place in the German top-flight, Sammer reckons the squad's lack of maturity will go against them.
Sammer said: “This is Borussia Dortmund, where people think: ‘We are already the German champions’.
“But they are immature. You could see it for some time now.
“Do all the players really know what they are up against? [Achraf] Hakimi does not, Sancho does not, [Dan-Axel] Zagadou does not. They do not know.
“Those three, plus ten kilometres less running overall and you have a match where you play with four players less.”
Sammer went on to say the lack of maturity in the Dortmund squad will go against them, insisting it's imperative for any title winning team.
He added: “That's what makes champions. But you can't be a champion, when you're 19 or 20 years old.
"Sancho is 18… how is that supposed to work?
“Of course there are factors like tactics, positioning, spaces, but that match is decided in your head. More in the head than with your legs.
“Look at the team and the players I just mentioned and their potential.
"It is not always about reaching your full potential, but with 60 or 70 per cent it does not work.”
Sancho has been a shining light for Dortmund this season, scoring and assisting for fun since breaking into the first-team after leaving Manchester City in 2017.
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