Now 31, Boateng is viewed by many as a player who has never really lived up to expectations after initially impressing as a teenager at Hertha Berlin, but he is on the verge of the move of his career and can hardly believe it.
"This furthers the profitable relationship between Barcelona and Sassuolo, born this summer as part of the buying and selling of the footballer Marlon", read a statement on the Serie A club's website.
"I've got a lot of respect for this side and that is why I had to say my goodbyes in person".
As Barcelona sought for a player to come from the bench, they had approached Alvaro Morata but he was unwilling to play a substitute role, while Carlos Vela and Cristhian Stuani were also considered.
He's also played for Eintracht Frankfurt and Las Palmas in the last few years, but there were no signs of a potntial move to a giant European club.
Barca will come up against Sevilla in the Copa Del Rey on Wednesday evening, when Boateng could make his debut for manager Ernesto Valverde.
Boateng, who joined Portsmouth from Spurs in 2009 before going on to sign for AC Milan a year later, will be presented to the media on Tuesday morning. He has four goals in 13 games this term. Denis Suarez, despite Valverde's reluctance to let him go, is still keen on a move to Arsenal, while the club are open to offers for Brazil forward Malcom.