Cesc Fabregas had a home debut to forget for Monaco as Thierry Henry’s side were well beaten by Strasbourg.
The midfielder joined the French side from Chelsea earlier this month and impressed in his first game for the club – a 1-1 draw at Marseille.
However, Fabregas had a difficult game at the Stade Louis II with his error leading to Strasbourg’s third goal.
Defeat means Monaco are still without a home league win this season and second bottom of Ligue 1 with 15 points.
The game got off to a terrible start for the hosts when Naldo, also making his home league debut, was sent off in the seventh minute.
The defender was judged – after a VAR review – to have brought down Ludovic Ajorque as the last man.
Ajorque opened the scoring for the visitors five minutes later with a header before Adrien Thomasson nodded in a second.
Radamel Falcao gave Monaco hope when he scored after racing through but Ibrahima Sissoko restored Strasbourg’s two-goal advantage with a stunning curled finish after Fabregas was dispossessed in his own box.
Ajorque dribbled through the Monaco defence to make it 4-1 and although Stefan Mitrovic was sent off for the visitors, they still managed to add a fifth through Youssouf Fofana in injury time.