Nwaneri arrived at Signal Iduna Park on deadline day as cover after Giannis Konstantelias suffered a torn cruciate ligament in his right knee, joining on a season-long loan from Arsenal that carries no purchase option.
According to Kovac, Borussia Dortmund had tried to sign the 19-year-old the previous year before eventually moving for him in the closing hours of the summer window.
"He's already shown in his two appearances that he can help us. I'm very, very happy with him. He'll be very important for us in the coming weeks and months," the Croatian head coach said at Friday's press conference.
Nwaneri has settled quickly at his new club, according to Kovac, who described the England youth international as having integrated "superbly."
"He's a fine lad, very calm, down to earth, friendly, helpful and very good with a football," Kovac added.
The Hale End academy graduate, who became the youngest player in Premier League history when he debuted for Arsenal against Brentford in September 2022 at just 15 years and 181 days old, made 37 appearances in all competitions during a breakthrough 2024-25 campaign at the Emirates.
He has already featured off the bench in both the Bundesliga and Champions League since his move to Dortmund, setting up an own goal in a 3-2 win over Hoffenheim before playing a part in a 3-2 Champions League victory over Villarreal.
Nwaneri's route to Dortmund was far from straightforward. He was omitted from Arsenal's Community Shield squad in August, with head coach Mikel Arteta making clear he would need to move on if he wasn't guaranteed regular minutes, but reports at the time suggested the youngster was reluctant to move abroad again following a difficult loan spell at Marseille last season.
He eventually joined Dortmund regardless, with the deal reportedly including a mid-season recall clause that would allow Arsenal to bring him back to north London as soon as January if needed, a condition understood to have been non-negotiable for the Premier League champions given their light attacking options.