Harry Maguire and Lisandro Martinez both missed the game due to suspension.
Ange Postecoglou’s Spurs climbed to eighth in the table on 10 points after six games.
United have several injury issues to deal with and have confirmed Sergio Reguilon will be absent against Crystal Palace on Saturday.
Eric Ten Hag’s side could not build on their first win against Wolves, bowing to pressure from a better Spurs side.
In his post announcing the news, the former Leicester City defender said he is “extremely disappointed” with the manager’s decision.
It was the Red Devils’ fourth victory over Nottingham Forest across all competitions this season.
Sunday’s win positions Arsenal as a serious contender for this season’s title, having gained 50 points in 19 games.
A controversial penalty handed Real Sociedad their first win against an English club in Europa League.
Ancelotti wished Casemiro well, saying the club cannot stop the player’s wish.
