Last year's Bundesliga runners up looked sluggish in a deserved defeat to basement club Darmstadt on Saturday and, with any title aspirations seemingly gone, Tuchel admitted that a certain lethargy and complacency had crept in to his team in the league. But his players "come in to their own in a 50-50 game," the 43-year-old said.
"We're finding it really hard ... to show this sharpness in these games away from the big stage, away from the spotlight," he continued, after Saturday's defeat left his team 15 points behind leaders Bayern Munich.
Dortmund will be hoping to find some relief from their domestic disappointments in a competition that has been kind to them this year. They finished ahead of Real Madrid to top Group F and scored a Champions League record 21 goals in the process. Their tally was significantly aided by an 8-4 win over Legia Warsaw that was the highest-scoring game in the competition's history.
Tuchel shuffled his pack for the Darmstadt match, naming three teenagers in his starting lineup and leaving Andre Schürrle, Ousmane Dembele and Gonazalo Castro on the bench. On Tuesday, Dortmund will be without Mario Götze, who hasn't travelled to Portugal but fullbacks Lukasz Pisczek and Marcel Schmelzer are back in contention after missing the loss at the weekend.
Though BVB have impressed in the Champions League this season, they haven't made it past the last 16 since 2013-14 and, with the German Cup their only other chance of silverware this term, the competition has taken on even greater significance than usual.
Dortmund last played Benfica in the 1963-64 European Cup where a 5-0 first-leg win in Germany took them through despite a 2-1 away defeat. The Portuguese side have not won any of their last four meetings with German sides.
Rui Vitoria's men have won the last three Portuguese titles though and lead the standings this season. They also have key striker Jonas back and firing after he missed the group stage with a knee injury. The prolific Brazilian scored 32 league goals last term and has struck, on average, once every 125 minutes in the Primeira Liga this time round.
Mexico international Raul Jimenez is also back in action and is wary of the challenge German side pose.
"It is going to be difficult as any Champions League match. We have to be at our best," the forward said.
Paris Saint-Germain host Barcelona in Tuesday's other last-16 first leg. On Wednesday, Arsenal travel to Munich to face Bayern and Real Madrid host Napoli.