I loved how pissed off he was about the shitty wall for that Mahrez free-kick... you tell 'em Dave.
He definitely needed to get on the four players in the wall, but Dave would have been as culpable for the goal as the wall would have been.
He set the wall up incorrectly. The tallest defender in the wall should be positioned on its near post end; as the wall extends towards the middle of the pitch, the defenders should be getting shorter. The main reason for this set up is to attempt to make a near post shot all the more difficult to get up and down. As the wall is extended, the shorter players will offer the GK the ability to see a shot that comes over the inner wall all the more quickly. The shot would only have been seen a split second earlier, but a GK like Dave with phenomenal reflexes can take advantage of that.
In the same vein as tallest player being placed at the wall's near post end, you want your biggest player in that spot (Ibra is someone who should always be there). The Mahrez free kick came through the wall at lower chest height due to Ibra jumping to the side, pushing Herrera and Rashford to their right, allowing for a large gap between him and Mkhitaryan. Placing the tallest (who is often one of the biggest players) makes it more difficult to jump sideways like Ibra did due simply to the fact that there are three players to his inside. The wall might get compacted, perhaps enough to allow a near post goal, but, more often than not, the wall will maintain its integrity by employing the correct players in the correct positions.
Just a very small issue about which to offer criticism, but it may be the little things like this that might keep us from winning cups or finishing fourth or better.