Arsenal manager Arsene Wenger admits he was surprised after former Barcelona winger joined Chelsea and not Manchester United.
The 28-year-old has agreed a four-year deal and is Chelsea's second signing in less than a week after the arrival of left-back Baba Rahman.
He made three official appearances for the Spanish giants this season before making the move to Stamford Bridge.
He could be in the starting line-up for Sunday's away game against West Bromwich Albion.
"It was a surprise to me," Wenger told reporters on Friday.
"I thought he was landing at Manchester but he finally landed in London.
"The story was out for such a long time that I thought he was going to go to Manchester, but then suddenly in 24 hours [he joined Chelsea].
"We've seen that before on the transfer market. I don't know what happened exactly.
"No [we made no move for him], we have many players on the flanks. Many, many players."
