1. His United team are not fluid. The movement should be systematic. X player occupies A space, therefore, Y player ought to occupy B space. When I watch Spurs, Chelsea, City, etc, I see a general tactical cohesion where players don't hesitate on the ball. I watched Matic on the ball for five seconds position his body towards Jones, waiting for Jones to run into a space to play in front of him. By the time the pass reached Jones' late run into space they'd reorganised and Jones smashed it up the field. Our lack of cohesion in movement hampers our momentum.
2. His United team sit deep in a block and rely on individual traits to make his teams offensively potent, however, Lukaku doesn't hold the ball up half as well as we'd like since he :-
a. Doesn't possess good ball playing technique.
b. Doesn't use his big frame as well as he ought to (see Kane, who is smaller, not as strong physically, yet more effective in his body positioning/shielding.)
c. Is prone to poor decision making when he has time, as opposed to playing on instinct.
We're isolated because of both of those two points. Poor off-the-ball movement, poor defence to attack transition (counter-attacking and transition are two different things).