Alcacer spent two full seasons at Barcelona as Luis Suarez’s understudy after joining from Valencia but left the Camp Nou at the start of the 2018/2019 season for regular first team football at the Signa Iduna Park.
Alcacer made 37 La Liga appearances for Barcelona, most of them as a substitute, and scored 10 goals but his time in Germany has been more prolific, notching 23 goals in 30 Bundesliga matches so far.
Ahead of Tuesday’s game at the Signa Iduna Park, Alcacer, who has five goals in four games this term, has shed light on some of his experiences at the Camp Nou.
"I'll decide in the moment," Alcacer told SER Catalunya when questioned on whether he would celebrate scoring.
"I have respect for many people there. Many people behaved very well with me but many people behaved very badly. My team-mates and the fans treated me well.
"As in any job, if people trust you, you give your best, and if they don't trust you, you don't give it.
"It's very important for me to see those signs of trust and in the end many people did not show it."
Lionel Messi is a doubt for the game after missing Barcelona’s opening four La Liga matches through injury, and Alcacer believes it will be a positive outcome for Dortmund if the five-time Ballon d’or winner is not passed fit to feature.
"It's clear that a player of that level is important for the team and for the club,” Alcacer said of Messi.
"You will always ask for his opinion and listen to him.
"Hopefully he will play next weekend, against Granada, and not against us!"
