That is according to Sky Sports reporter Kaveh Solhekol, who says Chelsea have until next Monday to seal a deal for the 28-year-old.
The Blues missed out on Lukaku after Manchester United stole the Belgian from under their noses.
Chelsea wanted to sign him but United's frustration at a lack of movement over a deal for Real Madrid's Alvaro Morata meant they switched their attention to Lukaku, who joined from Everton for £75million a week ago.
Aubameyang is now the No 1 target for the Premier League champions and he would replace Diego Costa if he arrived, who has been told he can leave Stamford Bridge.
How Chelsea could look with Pierre-Emerick Aubameyang Sat, July 15, 2017
How could Chelsea look next season if they sign Pierre-Emerick Aubameyang?
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How Chelsea could look with Pierre-Emerick Aubameyang
Kaveh Solhekol says Chelsea have chosen Pierre-Emerick Aubameyang as an alternative to Romelu Lukaku
Chelsea want to sign Pierre-Emerick Aubameyang after missing out on Romelu Lukaku
Kaveh Solhekol on Chelsea target Pierre-Emerick Aubameyang
Aubameyang wants to leave Borussia Dortmund and Chelsea will get their man if they cough up £70million says Solhekol.
"Chelsea want to sign Pierre-Emerick Aubameyang after missing out on Romelu Lukaku," Solhekol said on Sky Sports News.
"Dortmund will let him go this summer for about £70million.
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Pierre-Emerick Aubameyang wants to leave Borussia Dortmund and they will let him leave
"He's told the club he wants to leave.
"Dortmund are warning interested clubs they are running out of time.
"They want Aubameyang to sort out his future by next Monday. The clock is ticking."
Aubameyang has scored 85 Bundesliga goals for Dortmund in 128 appearances.