Riccardo Calafiori returned to Italy with Arsenal this week, and compared with when he left Serie A for England two years ago, he is already a completely different player.
Two years of time, with changes beyond just the environment

The original article does not provide specific statistics or evaluations from others, but only points out one fact: the player has undergone significant changes during his two years in the Premier League. After returning to Italy, he is no longer the same version of himself who left.
Where did the claim of 'the league's best left-back' come from?
The phrase 'the league's best left-back so far' in the title is not expanded upon in the original article with a specific source or quoted person, so it can only be retained as title information and not extended in the article body.