Detourbet scored three goals and registered six assists in 33 league appearances for Troyes last season, helping the club win promotion back to Ligue 1. He joined the Troyes academy at the age of eight and progressed through the ranks to become one of the most talked-about young players in Ligue 2.
Troyes confirmed the transfer on their official channels and paid tribute to a player who has been part of the club since childhood.
"Troyes is particularly proud to have supported Mathys' growth both as a player and as a man," the club said in their statement.
The loan to Monaco is expected to last one season, with an option for a second year that would activate if Detourbet plays five matches for the Ligue 1 side. The fee of around €25m, reported by L'Équipe, makes him one of the more expensive teenagers to move through the City Football Group network.
Manchester City retain priority on the player's future throughout the loan arrangement. The deal is structured to give Detourbet consistent top-flight experience in France before he is integrated into the City squad under Enzo Maresca.
Troyes' club statement is the only official comment on the move. The deal represents a significant bet on a player who has yet to prove himself at the highest level, but whose underlying profile has convinced both City and Monaco that the investment is worthwhile.
Troyes' promotion adds further context. Detourbet was one of the key figures in a side that dominated the second tier of French football, and his ability to contribute directly and creatively at that level has convinced City's recruitment team that his ceiling is considerably higher.
The Monaco loan is designed not just to maintain his rhythm but to expose him to Ligue 1 and potentially European football before the next step.