I'm sorry, he just stood and did nothing?
Are we seeing the same game?
Dalot and Smalling are the only ones (1) hurrying back to defend.
It was a deep run by Murphy from our left midfield whom Pogba should've marked, or if he fail, Pereira to takeover. I don't get why Smalling is too far near the touchline trying to help Young defending against one player, possiblly Ole instructed Chris to do that. Anyway, McTominay and Dalot are the only left defending the box. McTom heavy-marked his men well to the ground, but no one else marked Murphy.
Tbf, it's true that Dalot should've stick to his man (Mendez-Laing) instead of (2) trying to cover by moving towards Murphy (since he's too far, Dalot won't make it, leave it to De Gea and others) then when he couldn't make it, he decided to (3) try to intercept with a slide which failed. It's a quick attack.
Lack of experience and good defending coaching in his decision making.
So Dalot did 3 things.