Who knows? Could be the pressure of his price tag getting to him? Could be he’s struggling to settle in a new league and a new country? Maybe both of these things combined with the hostile crowd and the absolute shit-show he’s walked into have shattered his confidence?
If I had to guess I’d say it’s because he maybe isn’t putting enough effort in defensively and he’s maybe a little too one-footed, which makes his actions too predictable. We know Arteta is demanding max effort as a ‘non-negotiable’, and from the training clips I’ve seen it looks like we’re putting a lot of work into drilling the players to receive the ball and play a pass in 360°, so perhaps he’s flunking both these tests at the moments (or simply isn’t doing them as well as Nelson)? Again, it probably comes down to confidence.