The 25-year-old striker joined the Catalans from Valencia in 2016 but made only 23 league appearances in two years at Camp Nou, struggling to displace the likes of Luis Suarez and Lionel Messi.
After signing for Dortmund on a season-long loan deal in August, Alcacer impressed so much that the German club announced in October they would make it a permanent switch for €23 million (£20m/$26.5m).
Speaking to Sport Bild, Alcacer said: "Since joining BVB, I'm finally feeling important again. I've played very little in Barcelona for the past two years. That's why I understand that I haven't always played from the start so far.
"Every footballer wants to play. I'm not happy when I'm sitting on the bench. But if we're having so much success as a team, it's OK. Because getting a title is always great.
"I only have one thing in my head: I want to score goals and help the team. This has worked perfectly in the past few months.
"Actually, I've been a little surprised with the ease that I've been scoring the goals with. But I have to say that my teammates have a big part in it. They often make it easy for me to score.
Alcacer is the joint top scorer in the Bundesliga, tied with Frankfurt's Luka Jovic on 12 goals despite playing half as many minutes as the 21-year-old.
In fact, Alcacer has made just 12 appearances in the league and only five starts after being hampered by injury problems, although a substitute role has suited the forward with 10 of his strikes coming from the bench.
"If you suddenly have to play every three days after not playing regularly for two years, that's a big change for the body. I was aware of that," Alcacer continued.
"But I work with the coaching team to get my body up to 100 per cent efficiency. I want to avoid injuries in the second half of the season."