Man United manager Erik ten Hag admitted his side displayed a horror show in the opening minutes of the match.
The Red Devils are now back to the European top tier competition after missing out last season.
It was the Red Devils’ fourth victory over Nottingham Forest across all competitions this season.
A statement from the club on Tuesday said the Denmark player was “likely to be out until late April or early May”.
Arsenal failed to extend their lead atop the Premier League table after they were held to a goalless draw by Newcastle United at the Emirates Stadium.
A controversial penalty handed Real Sociedad their first win against an English club in Europa League.
The Brazilian star departed Madrid on Monday after his farewell speech.
Ancelotti wished Casemiro well, saying the club cannot stop the player’s wish.
Sources said Will Smith had gone to India to seek healing and undergo therapy over the incident with American comedian Chris Rock.
Eriksen, 30, spent half of last season on loan at Brentford, making four assists and scoring one goal in 11 games.