The versatile defender made his first-team debut for the Blues in a League Cup win over Bristol Rovers in 2016 having progressed through their Academy.
Among his trophies won with the club’s younger sides are two UEFA Youth Leagues and two FA Youth Cups.
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Aina spent last season on loan with Hull City in the Championship where he played on a regular basis for the Tigers.
The Nigerian International will now continue his development in the Italian top flight.