The young goal-getter is considered one of the hottest prospects in European football, starring for Viktoria Koln and Leverkusen in recent seasons.
He even played for the Blues at youth level during a couple of friendlies earlier this year.
But the Austrian U17s star now appears to have made the move permanent, according to Austrian outlet 12termann, arriving at Stamford Bridge with a burgeoning reputation.
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Born in Guinea but raised in Europe, the talented striker joined Viktoria Koln from Bayer Leverkusen in 2016 and has scored seven goals in 25 league appearances for the club’s Under-17 side.
Former youth team coach Peter Huemerlehner says Ballo scored over 200 goals in two years at his first club SC Chemie Linz.
However, Chelsea fans saw little to smile about after the news made its way onto social media site Reddit earlier this week.
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“I can't wait to see how well he does in his 5 consecutive loans before we sell him for 20mil in 2023,” one wrote sarcastically.
Another joked: “Premier League title here we come.”
It comes amid reports that under-fire Real Madrid boss Zinedine Zidane has requested the signings of Eden Hazard and N’Golo Kante as he seeks to reverse Los Blancos’ hideous domestic form.