The Spanish international, 27, had the opportunity to move back to the Emirates this summer after new Barcelona chief Luis Enrique told him he was surplus to requirements at the Nou Camp.
However, he turned down the chance to rejoin the north Londoners and joined the Blues instead.
His decision to snub his former team has perplexed many, including his new manager - who has admitted he doesn't know why he turned down the Gunners.
“I know why he chose Chelsea [but] I don’t know why he didn’t choose Arsenal,” Mourinho said.
"I don’t want to speak about this.. About what we promised him about what he promised us.
"I cannot speak about his time at Arsenal, the decision that made him leave or the decision that stopped him to go back.
"I don’t know if it was the manager’s decision or if it was his decision. I really don’t know.
“I just know that we promised him that we were going to build a team [that was] very adapted to his style of play.”
