)
Reus limped off on 44 minutes of Dortmund's 6-2 rout of Bayer Leverkusen in the Bundesliga with a suspected tear in his left hamstring.
"It's sad news, he (Reus) has been playing very consistently for weeks and at a high level," said Tuchel.
"It's an enormous loss which tarnishes the win."
Dortmund are looking to overturn a 1-0 defeat from the first leg at Benfica.
)
)
)
The 27-year-old Reus missed Germany's 2014 World Cup campaign after tearing ankle ligaments in a warm-up match before Brazil and then missed out on a place in the squad for Euro 2016 with a pelvic injury.
With Reus sidelined, the pressure will be on star striker Pierre-Emerick Aubameyang to get the goals to put Dortmund in the quarter-finals of Europe for the first time since 2013/14.
The Gabon hot-shot now has 21 league goals to make him Germany's top scorer this season after he scored with a well-taken shot and second-half header against Leverkusen.
But he could have landed in hot-water with club bosses for having the logo of outfitters Nike shaved into his hair cut -- a potentially embarrassing move for his club Dortmund, who are sponsored by Puma.
"If I am interpreting that correctly, that must be his personal sponsor," said Dortmund's director of sport Michael Zorc.
"That can't be repeated often. We'll talk about that."