The first point is basically Fred, he has to play instead of Fellaini imo. He's more dynamic, but tiny so Fellaini starts ahead of him. Fred is quick, moves forward and moves the ball with ones touch.
Second point - nobody compares Rashford to Mbappe anymore, and haven't done for awhile. Both Lukaku and Rashford are clearly good players, its just not working in this team. Tactically - we don't work to Lukaku's strengths - we should be following the way Spurs are built around Kane. He should be having 2/3 good heading opportunities a match minimum.
Valencia isn't the problem - sure he wasn't great last night, but it was one game - doesn't look unfit yet at 33. Dalot is quite attacking so i feel he would be far more exposed.
Last point is fairly common knowledge - Smalling can do the job decently enough, but none of the others can partner him properly. Ashley Young is the worst player in the team this season for me.
For me, Mourinho has got to stop this 'fear football' - putting out a team to stop the opposition rather than to play our own game (ie the West Ham lineup/formation). Work on moving the ball faster and taking more risks.