He had been in decline for years
16/17 he was up there as one of the best fullbacks in the league. Walker talked of Valencia as one that inspired him in terms of consistency. That season he would still outpace players like Sane.
He started the season of 17/18 ok, but then faded badly. To me, it looked like he carried an injury, and there have been talks about a knee injury. I'd say his decline started last season, and it looked like he had to play through injuries, much like Evra in Fergie's last season.
This year, he has been well below par when played. He still looks fast enough when he gets going, but his engine is not what it was, and he's lost agility, which wasn't great to begin with, again the knee issue.
As for the taking on players bit, I've argued many times that very few fullbacks actually do this. Valencia and Evra spoiled us in that regard.