Their start to the new season has been more or less ideal, with a decent amount of goals being scored, and their defensive record being the best in the League.
United are on top of the league table after four matches, scoring 12 goals.
"But Manchester United are really strong, a stronger team than last season and at the moment we're not at the level we were last season".
United are huge favourites to win the game at 4/11 odds. Pogba will be out for "a few matches".
Koeman failed to land an out-and-out replacement for Lukaku in the summer, with Wayne Rooney the only player to have found the net for Everton in their opening four games, and Koeman accepts Dominic Calvert-Lewin and Sandro Ramirez can not be asked to assume Lukaku's role.
Koeman said: 'I am anxious about our aggression on the pitch and how we defend. Luke Shaw, on the other hand, recovered from his injury and will be available for selection.
He has now scored six goals in as many games since his arrival from Everton. Even Roberto Martinez at Belgium has given Fellaini a platform on the worldwide stage having once questioned whether or not he'd be suitable for his kind of football at Everton.
Every Evertonian knows just how good that man Lukaku is in front of goal, having bagged himself 87 goals in 166 appearances in the royal blue.
James McCarthy is a doubt due to a knee injury.
Everton are set to be without long-term absentees Ramiro Funes Mori, Seamus Coleman and Yannick Bolasie.
The Belgian has scored ten goals in eight games for club and country so far this season, and Mourinho has been impressed.
Though previously put in the negative light with United fans from two transfer request during his time at Manchester, Giggs believes the fans will give Rooney a warm reception on his return to Old Trafford on Sunday. The Red Devils have 85 wins, 44 draws and 66 defeats in 195 meetings across all competitions. Wayne Rooney is also likely to start and will be hoping for warm welcome after securing legendary status at the club where he became record goal scorer.