The Barcelona ace admits the Hoops are a very strong club but suggests they could be even better, especially in the Champions League, if they make a point of holding onto their stars like Gary Hooper (now with Norwich City), Victor Wanyama (Southampton) and goalkeeper Fraser Forster who is being linked with a transfer.
"There is no doubting the size of Celtic as a club but if they want to improve in Europe they must keep their top players," the Spaniard said ahead of the Nou Camp clash with the Bhoys on Wednesday night.
"The performances of Wanyama last season meant he got a move to England and the same is true of Forster this season.
"It is a very difficult situation for them. If the players are not noticed it means they are not doing well in Europe but if they do well in Europe their best players get noticed.
"It is not an easy situation but they must try to find a solution."
The World Cup, European Championship and Champions League winner added: "I understand Scotland is not the biggest league in Europe but players should think about it before they leave.
"They might be leaving for a bigger league in England but the truth is they will probably not be leaving for a bigger club."
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