Borussia Dortmund coach Thomas Tuchel felt the injury suffered by Marco Reus took the edge off his side's 6-2 Bundesliga thrashing of Bayer Leverkusen.
Attacking midfielder Reus, who has featured just 15 times this season due to injury problems, was forced off in the first half of Saturday's win and is not expected to return until April, meaning he will miss the second leg of Dortmund's round-of-16 Champions League tie with Benfica on Wednesday.
Tuchel, whose team closed to within six points of second-placed RB Leipzig with the emphatic victory, told the club's official website: "It's a real blow. In the opening stages today, Marco was the one who constantly got us going, who took the responsibility, who was alert.
"He has been incredibly stable for weeks now, he's been playing at a high level and is an important character to have on the pitch.
"The fact he can't play on Wednesday is an enormous loss and takes the shine off the win."
Dortmund were two goals up at the time of Reus' withdrawal thanks to Ousmane Dembele and Pierre-Emerick Aubameyang.
Leverkusen twice pulled to within one goal following strikes from Kevin Volland and Wendell, either side of Aubameyang's second goal and 21st in the league this season, before substitutes Christian Pulisic and Andre Schurrle and full-back Raphael Guerreiro secured the points.
Defeat for Leverkusen sees them drop to ninth in the table and heaps more pressure on coach Roger Schmidt.