The 18-year-old remained at Dortmund despite interest from Southampton and several Italian clubs, including AC Milan, but his opportunities under Niko Kovac have been extremely limited.
Reggiani has made only two senior appearances this season, with one brief outing in the German Cup and another in the Bundesliga.
That lack of playing time is a concern for the Italy international, whose expectations increased after his performances last season.
According to the Ruhr Nachrichten, Reggiani is now regarded as Dortmund’s “hottest departure candidate” ahead of the winter transfer window.
Southampton’s interest shows that there is already a market for the centre-back, while Italian clubs have also been monitoring his situation.
Dortmund could therefore be open to allowing Reggiani to leave, particularly if the club decides that regular football elsewhere would be better for his development.
The situation is also connected to Dortmund’s own plans for January. The club is considering additions to the squad, with Marcel Regula reportedly high on the list of potential targets.
The 19-year-old attacking midfielder currently plays for Zaglebie Lubin and was already considered by Dortmund during the summer.
Regula could once again become a priority in January, potentially creating further movement within Kovac’s squad.
For Reggiani, however, the immediate question is whether he remains at Dortmund and waits for more opportunities or seeks a move that would give him regular first-team football.
His limited involvement this season stands in contrast to the expectations created by his previous campaign, when he established himself as a player capable of contributing at senior level.
Dortmund now have several weeks to decide how best to handle his situation.
A permanent sale would provide the club with a return on a player who has attracted outside interest, while a loan could give Reggiani the playing time he currently lacks without ending Dortmund’s longer-term involvement with him.
The interest from Southampton and Italy means his future is likely to become one of Dortmund’s key transfer questions when the January window opens.