The mistake still lies within our midfield.Actually I think it's a collective mistake.
Look how much lateral space is between our defenders. Add to this that no one is actually pressing Atsu so he has sooo much time to pick the right pass at the right time. An easy goal.
Another angle the moment Atsu makes the assist, huge space.
The moment Atsu made the pass no one is on him. Our defence is badly exposed and I noticed this in the goal Aston Villa scored, no one picked on McGinn's run because our midfielders were too advanced.
As you say there is zero pressure on the ball from Sissoko, it's fine to stand a player up, which is what I think he's trying to do, but he needs to be closer to either not make it possible for the cross to be attempted or block it if it's tried. If that's Dier, Dembele or Wanyamma than they will be aggressive in forcing the player on the ball out wide, they can afford to even attempt to tackle the player, maybe force him to attempt to dribble past them (if they fancy themselves) which will give the covering players (KWP & Toby) a chance to engage in the 2nd phase, there should be very little on for the player on the ball other than a ball squared (that should meet the same problem).
There are lots of things that can be more forensically dissected about the positioning of the players but all of that is completely unnecessary if we simply are more aggressive towards the player on the ball. Make them play a rushed pass and lose possession as a consequence or win the ball back by tackling them in the first phase or in the 2nd phase when he may have the ball under less control whilst taking on Sissoko.
It's quite basic, which is the worry.