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Football is a game of rhythm, both on the pitch and off. When a team is in full flow, as United was against Arsenal, it is usually a result of the players being able to play regularly together in order to maintain their understanding of one another.
The international break robs a team of, essentially, a whole week of training and one regular match day. In the long run, it's a small thing. But for a team like United, whose squad was almost picked clean by national teams this past week, such a long time away from teammates can be detrimental in that it breaks up the rhythm the team has so carefully built.
This is why, in my experience, games directly following an international break can be slow and sluggish. That break in understanding can be one factor. There's also the added pressure of actually trying to help your national team qualify for Europe or just hoping to provide a good result.
And then there's the 8-2 drubbing of Arsenal. On the one hand, the players were given an extra week to savour the win. On the other hand, United is a young team, and it is entirely possible that the extra week allowed some extra pressure to creep in, given all the press the scoreline has garnered. It's not improbable that doubts may have started to creep into the minds of the younger players; doubts like "can we do it again?" or "was it just a fluke?" If they let the extra pressure get to them, Bolton could pounce.