Gvardiol is great at CB. I'm not sure why he was pulled after Al Hilal scored other than perhaps Pep wanted a faster CB.
Al Hilal were killing us on the left - but that's almost entirely on Ait-Nouri.
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For their first goal, Ait-Nouri went into a tackle a bit too hard and the attacker went past him. Then Ait-Nouri took a bad angle - due to lack of pace? - and the attacker went far down our left flank unopposed.
Gvardiol was there in the box - which I think is his correct position.
Ait-Nouri, though, due to his poor angle, ended up right next to Gvardiol - covering an already covered position - and then, having reached this poor position, failed to close down the attacker.
A cross went in - unopposed - and Ederson made a mistake by parrying it forward - which happened to hit Dias - poor parry nonetheless - and in the mayhem that followed Al Hilal scored.
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For their second goal, Gvardiol is in Al Hilal's box - because we're taking a corner kick. In this situation, Ait-Nouri is our deepest defender.
The cross comes in... and then Al Hilal gains control. A fast pass forward is made... and Ait-Nouri, our deepest defender, is ball watching. Meanwhile an Al Hilal attacker to his right is sprinting full out. Ait-Nouri can't afford to ball watch - he needs to watch the field - and when we lose the ball - and an attacker is sprinting forward - Ait-Nouri needs to sprint back.
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IMO - Gvardiol did nothing wrong in the first two goals scored by Al Hilal - Ait Nouri was 80% to blame for their first goal (because Ederson could have parried the shot far to his right or left instead of centrally - and 100% to blame for their second goal - because he failed to read the game and was ball-watching as our deepest defender.
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In this game I've learned a lot more about Ait-Nouri - he's a great dribbler - and an attacking asset - but on defense he's awful. Can't read the game. Not enough pace. Ball-watches.
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TL/DR
If Ait-Nouri is playing - FFS don't put him as our last defender on corner kicks.
And if Ait-Nouri is playing - we're going to concede crosses into the middle - because Ait-Nouri lacks pace, and positional awareness to defend.