Played some of his best footy under Martinez.
Agreed, but only briefly and he became increasingly inconsistent in Martinez' 2nd and 3rd seasons, to the extent that he was eventually hauled off and left out, even by Roberto. Under The Golfer he had his most consistent spell, not brilliant but good. Souness is right in that article, he did things at Southampton that he never showed at Everton where, as he says, he too often beat a man, or even two, got to the edge of the box then lost possession or gave it away. Martinez never attempted to correct this obvious flaw in his game, or if he did, he failed.
In addition to the better coaching he seems to be receiving, I'm sure his spell in the 'wilderness' combined with watching just how hard Chelsea's better players work on and off the ball has made him more receptive to advice. Still got a long way to go to reach and stay at their level.