He's been playing brilliantly for most of the season just like the rest of them, there's no denying it.
But I still won't be convinced he's actually a brilliant player. He looks the same player he was for Spurs, to me - strong, rapid, potent in attack but just not clever enough to be a really good fullback. The initial mistake is one of those that even a great player can make, and they all do at some point or other.
But what on earth possessed him to pursue at that weird trajectory? Did he think Grigg was going to make a heroic dash for the corner flag? If you've got Walker's pace, you basically bomb straight for the goal, whichever side of the player that ends up taking you, and by the time he's within shooting distance you should at the very least be in a position to make a tackle, if not to block his sight of goal.
And maybe his excellent recovery pace has prevented him learning the dark arts of the position, too. A canny fullback will run across Grigg's back as he chases in that situation - gives you the chance to either end up goal side of him if you do catch up, or at the very least clip his heels during the chase and bring him down, possibly 'accidentally' enough to escape a sending off.