It's definitely something that's by design rather than our inability to do it.
We seem to play in an almost passive state for a lot of games (particularly away), where we would rather retreat and sit deep allowing our opponents to have the ball, as opposed to pressing high with the intention of luring our opponents into mistakes.
I always find that United make it easy for their opponents to play against them. Not in the sense that we lose a lot of games, or indeed concede many goals, but they will always allow their opponent to pass around freely and encroach into the final third, without ever really pressing with any sort of purpose.
This stye served us well at the beginning of the season, mostly due to the fact that we often got an early goal or certainly the first goal, which would then result in our opponents committing numbers forward, which would then allow us to counter late on in the game when they were tired. I think we've scored more goals in the last ten minutes of games than any other team in the league; that's obviously not by accident.
But this passivity starts to become detrimental when we don't get the first goal (Huddersfield and Chelsea). This is where Mourinho starts to show some of his flaws; he starts to run out of ideas, imo. Things start to become frantic and desperate. As we seen with the Chelsea game, we threw on Fellaini and started to launch the ball to try and cause some chaos. It's all a bit simplistic and obvious, imo.
I don't like this reactive, passive approach personally. I know it has served in very well in the past, and he's obviously a fabulously successful coach, but i'm not sure how well it's going to treat him now, or in the future.