As I saw it, we didn't drop that deep. It's also normal to get pushed back like this when you're one goal up against the home team. Burnley didn't play bad in the first half and found good spaces between our midfield and back line. After the goal we couldn't afford letting them do that, so Mourinho moved Pogba down a bit to help Matic with those areas.
But, for sure, we should have put this match to bed with a second goal. We had so many occasions where a lite more concentration and awareness would've gotten us that second and third goal. It's not Mourinho's fault the attacking players made the wrong decisions in those moments. It's all on them, and they know it.