He is only 21 and is progressing leaps and bounds. The proof is in the pudding: last season he scored 23 goals and got 6 assists in 52 matches. It was the year he became a 'Bhoy', as before he was with Celtic, but only on loan.
He is a product of the PSG youth system. After several attempts to catch Tuchel's attention through a series of loan moves, he didn't manage it and moved to Scotland permanently. They rushed to sign him as soon as they saw his quality as a striker.
He has always been in Mbappé's shadow, so it makes sense that the French giants let him go for 10.3 million euros, taking into account that he wasn't guaranteed game time with the first team.
He does have them under Neil Lennon and 'Sky Deutschland' have reported that Borussia Dortmund are tracking him. They say that the Germans could make an offer for him in the January window.