Paco Alcacer on Thursday became the victim of Erling Braut Haaland's spectacular start at Borussia Dortmund as the Spain striker left to join Villarreal, the German club comfirmed.

The 26-year-old Alcacer reportedly cost the Spanish league side 30 million euros ($33 million), including bonus payments, after signing a deal until 2025 to return to Spain after only 18 months in Germany.

"We would like to thank Paco Alcacer for his performance in the Dortmund jersey and wish him good health and maximum sporting success in the future," said Dortmund's sports director Michael Zorc.

The injury-prone striker joined Dortmund on loan from Barcelona in August 2018 and made a flying start with seven goals in his first four games.

His transfer was made permanent last February when the German club paid 23 million euros for him, but his form has since dipped sharply and he has been blighted by injury.

His prospects of holding down a starting place nose-dived after Norwegian wunderkind Haaland, 19, scored five goals in his first two games since joining last month from Salzburg for 20 million euros.

Alcacer's transfer means money is immediately available to sign Emre Can from Juventus.

With the transfer window closing Friday, Dortmund are trying to bolster their midfield by signing the Germany midfielder as cover for injured Danish enforcer Thomas Delaney.

Earlier, head coach Lucien Favre was cautious as "no final decision has been taken" for 26-year-old defensive midfielder Can, who has made 25 appearances for Germany.

German magazine Kicker predicts Can's transfer would be worth around 23 million euro to Juventus.

Dortmund host mid-table Union Berlin on Saturday looking to avenge "the bad memory", as Favre called it, of their shock 3-1 defeat in the capital when the clubs met for the first time in Germany's top flight last August.

Dortmund fans hope Haaland will make his first start at Signal Iduna Park with Favre commenting "it's possible, we will see".

Haaland says that he is fine with his current role as a sub, "when I am on the bench, I'm just more motivated to deliver".