What was Dier supposed to do? Tavernier went past Sanchez like he wasn't there, should Dier have stayed with his man and let Tavernier straight in on goal with a relatively easy finish or go and try to close him down? Porro made zero effort to get back when he could see Sanchez was in trouble, it was only Dier reacting. There's no way Dier could've predicted Sanchez toe-poking the ball past him.
If he didn't try and close him down and Tavernier scored he would be getting, rightly, slated for it.
Dier needs to stay with his man, and hope Sanchez and Loris can deal with their man.
The moment you leave your position....or you don't read the danger nearest to you...and you go off to fight someone else's fire, you mess up the defensive structure for everyone.
Dier's job is not to chase player who has the ball and is likely to play score a goal, his job is to maintain the defensive structure. It's the structure which stops opposition from scoring...not the individual defender.
Dier always panics and breaks the structure, that's why he is a really brick CB.
You always get these bang average defenders who are bigger and stronger than everyone else, but can't read the game. They think they should be some kind of superhero and prance around the pitch trying to act like one. The problem is they never work as a team player in backline unit.
Their incompetence makes other players look bad.
Dier has good games and very bad games, he is a Premier league level Defender, but he will never be a top 6 Level defender consistently.
Davies and Romero are far better at reading the game and following system structures... Don't get me wrong they have brainfarts too. But you can see they understand that defenders have specific roles.
You have to win your individual battles (that could be anything from beating your man to the ball, or just standing next to an attacker so he can't make a run).
What you shouldn't do is leave your man and go chasing the ball.... not in Defence...He could do that as midfielder and get away with it....but not in Defence.