His legs have totally gone. Playing 4 summers on the spin from 2014-2017, should be as the main cause of this, especially with his style of play, playing in the physically demanding PL and longevity as a player, this is 14th season as professional and he isn't even 30 yet.
He should be seen as a cautionary tale, to show why playing tournaments every summer, with the players getting minimal rest, between the end of one season and the start of another, will see more players end up, having a sudden decline like this. His decline is similar to Rooney's, started playing 1st team football at a very young age and along with his high energy style and chasing down lost causes, when he finally did hit the so called athletes peak in the late 20's, it actually was the start of his decline.