I don’t think his performance drop.
It’s rather defenders learn how to contain him, it’s very easy, to be honest. His runs or so call dribble is always in a straight line with no twist and turn or change of direction. All they have to do is to follow his run and exert a tad of physicality and he will lose the ball and ends up barging into defenders.
Lack of dribbling ability is the main reason. He is basically playing the same way as he did at the start of season, the only difference is defenders learning his flaws and he had no answer to it.