He has some value as a wing back but every time he starts in a back four, I die a little inside.
Watch their goal again. Martina chases the ball to the edge of their defensive third, the defender slips a pass around him, and Bournemouth have an instant advantage going forward. Holgate, Schneiderlin, and Williams did poorly during that sequence but it's a big ask for them to cover all of the space Martina vacated in hopeful pursuit of the ball. If Jonjoe Kenny was that clueless tactically, he wouldn't be picked for the England U20s, let alone Everton.
He's Koeman's player but I'm blaming Steve Walsh for this. If he has as much influence over transfers as we're led to believe, he should have said "absolutely not" when Koeman brought him the name. You only have to watch a handful of games to see that this guy can't defend.