Barcelona announced the signing of Frank Kessie and Andreas Christensen on free transfers.
Ivorian midfielder Kessie, 25, joins from Milan, while Danish defender Christensen, 26, joins Chelsea.
The two players signed deals for four years and the terms of their release were set at €500m (£430m).
Casey played 224 matches for the Rossoneri team, scored 39 goals, and helped the club achieve their goals The first Italian league title At 11 years old last season.
Christensen came through the youth academy at Stamford Bridge and played a total of 161 games for the Blues, while also spending two seasons on loan at Borussia Moenchengladbach.
He won the Champions League, European League, European Super Cup and Club World Cup with Chelsea.
Brazilian midfielder Philippe Coutinho left the Nou Camp this summer, joining Aston Villa on a permanent deal for £18m, while striker Ousmane Dembele and right-back Dani Alves left after their contracts expired.
Meanwhile, Manchester United has They reached a broad agreement More than €65m (£56m), plus extras, are with Barcelona for Dutch midfielder Frenkie de Jong.