Haller, who scored 34 goals for Dutch champions Ajax Amsterdam last season, was diagnosed during a pre-season training camp in July - weeks after he had been signed for an initial fee of 31 million euros.
The 28-year-old returned to training earlier this month and finally made his unofficial debut as he came on in the second half of Tuesday's friendly with Fortuna Dusseldorf, which Dortmund won 5-1.
"It was a great moment on the pitch. I got a lot of applause and a lot of messages, I spoke to my teammates and the opponent. This support contributes to performance," Haller said.
"I wanted to be there from the first game. We talked about what is the best solution to get some minutes. After six months, I finally wore a Dortmund jersey for the first time.
"I was really looking forward to playing because I couldn't do it straight away after my move. Playing is a much better feeling than exercising and running through the woods, I missed that feeling."
Dortmund coach Edin Terzic said the team were proud of Haller's comeback.
"It was a very special moment for Seb and his family, but also for us. It was the first time that he wasn't wearing a training shirt but the Borussia Dortmund jersey," Terzic said.
The Bundesliga season resumes later this month, with Dortmund taking on Augsburg on 22 January.
Dortmund are sixth in the standings - nine points behind leaders Bayern Munich - and teammate Gio Reyna said he hoped Haller's comeback can spur them on.
"We are happy that he's back. He's such a good guy, he works hard," the American winger said.
"You can see his quality immediately on the pitch. He gives our game something new."