Borussia Dortmund coach Thomas Tuchel has outlined his concern at Pierre-Emerick Aubameyang's underwhelming form over the second half of the season.

The Gabon striker remains top of the Bundesliga goalscoring charts with 17, but has netted just once in six appearances in all competitions since his return from the Africa Cup of Nations, with a missed penalty in the Champions League defeat at Benfica highlighting his difficulties.

He recently admitted fatigue is damaging his form and Tuchel has acknowledged Aubameyang's struggles could cost Dortmund in their bid for Champions League qualification.

"Every slight dry spell of Aubameyang worries me," the Dortmund coach said at a news conference ahead of this weekend's match against Freiburg.

"He is incredibly important, we depend on him and will not achieve any of our objectives without him. 

"If Aubameyang is in top form, we are confident of scoring goals at every given moment."

Dortmund will still be without Mario Gotze and Sven Bender when they take on Freiburg, with the latter's recovery from an ankle injury going slower than expected.

"We have to accept Bender's recovery is taking longer than we would have liked," Tuchel added.

"I would not say he has endured a setback, but he is still experiencing some pain."