Dortmund enter the second leg at White Hart Lane with a 3-0 advantage, and one foot in the quarter-finals.
In addition to missing players through injury and suspension, head coach Mauricio Pochettino made several unforced changes to the line-up which started the North London derby.
Reus told German channel Sport1 he understands Tottenham's decision, but is confident Dortmund could have beaten their top team anyway
He said: "I do not believe that they have taken the game seriously. We are currently in such a good phase that we could have beaten in their best side, but each team has their own priorities.
"I think Tottenham gives priority to the championship in the Premier League, which you can not blame them."
Reus played a crucial role in Dortmund's win, netting a brace, and he believes it was a real statement of a performance which they must look to repeat.
He said: "If we play like against Tottenham in the first leg, we can achieve something. If we give a few percent, we have no chance."
Dortmund are like Tottenham, looking set to qualify for the Champions League next season by virtue of their domestic record, in second in the Bundlesliga and pushing Bayern Munich harder than over the three previous years.