One hundred and one days have passed since Paris Saint-Germain beat Arsenal on penalties to retain their Champions League crown, with only a World Cup and a record-breaking transfer window to keep us entertained since.But Europe’s elite competition returns on Tuesday with six group-phase fixtures — and 12 more before the end of the week — including four debutant teams and many of the players who lit up the summer with their performances on the international stage.With the expanded format, the Champions League has also given some of the continent’s most promising young players a platform to shine. By now, we are a little more familiar with Lamine Yamal, Desire Doue, Nico Paz, so who are the next group of players — aged 23 or under — who will be vying for our attention?The Athletic’s data and tactics team run through some of their favourite picks.Konstantinos Karetsas (18, attacking midfielder, Borussia Dortmund)Konstantinos Karetsas walked out in front of more than 80,000 people to make his Bundesliga debut for Borussia Dortmund in late August. Still just 18 years old, he took more shots (six), attempted more take-ons (six) and received more passes in the final third (27) than any other player on the pitch.A zippy attacking midfielder bursting with confidence and flair, Karetsas has wasted no time endearing himself to his new fans. He picked up 10 assists at Belgian side Genk last season, a constant buzz of energy across the width of the box, and he has already opened his creative account for Dortmund with a sharp one-two to set centre-forward Serhou Guirassy away.The young Greek is happy to take aim from distance, and strikes through the ball cleanly when he can chop inside onto his left foot. Here he is scoring for Greece with what you would already call his signature finish, a curling finish high into the far corner.