Germany international Reus has been dogged by fitness woes over recent seasons and was making only his 11th Bundesliga appearance of the campaign when he departed with a thigh problem after 44 minutes of Saturday's 6-2 win over Bayer Leverkusen.
Tuchel acknowledged the setback as a significant blow, stating the 27-year-old was playing at close to his best over recent weeks, and he is still mulling his options as Dortmund aim to overturn a 1-0 first-leg deficit against the Portuguese champions.
"I cannot tell you how we will replace Marco Reus," he told a pre-match news conference.
"For one I don't want to, but I also don't quite know yet with which player type we will line up where. But there is no doubt, he is a loss, he was in outstanding form."
Pierre-Emerick Aubameyang had a penalty saved as Dortmund failed to find a way through Benfica's stubborn defence at Estadio da Luz.
"We need to free our minds from the 1-0 scoreline. It won't be at the centre of our preparations," Tuchel said.
"We've got the feeling that we're able to score in different ways, we're able to prevent counters, able to beat Benfica.
"Benfica is not only a big name. The aura of the club always rubs off on the players, who take confidence from that.
"We have the feeling that we can progress, we're feeling that it's the right moment for it as well.
"Tomorrow's match is the kind of game everyone looks forward to when he signs a contract at Dortmund."
Left-back Raphael Guerreiro is a doubt for Dortmund having sustained a muscular injury at the weekend.