The striker has been a huge hit in the Bundesliga since joining on an initial loan deal in August, becoming only the second player after Gert Dorfel to score six goals in his first three games in the division.
Alcacer’s impressive run, which included scoring a hat-trick as a substitute in last Saturday’s dramatic 4-3 win over Augsburg, earned the 25-year-old a recall to the Spain squad for this month’s matches with Wales and England.
The former Valencia man admits he had reached a point where he knew he needed to leave Barca, having made only 14 starts in LaLiga in two seasons.
“I decided to look for minutes elsewhere. A player with confidence is better than one without confidence. I left in order to feel happy playing.
“The decisions I was able to make, I made. The conversations with [coach] Ernesto [Valverde] were private and I decided to look for minutes elsewhere. At Barca, with Luis Suarez, [Lionel] Messi, [Ousmane] Dembele… it’s tough to play.”
Alcacer credited his Dortmund form to the fact his new team-mates have helped him to settle quickly.