Not on what basis but the statistics are far different, Ansu fati will not be equal to Messi, maybe only other opinions say so but if he is loaned to another club it means that his performance is bad at Barcelona.
It will be difficult to find another player who is as brilliant as Messi at a young age until now even more top award winners including all his goals, so I think Ansu fati is only in the class of other players, even if he returns to Barcelona and there is no change he will probably be sold.
So it is clear that Xavi's alibi regarding Ansu Fati is only about the selling price which does not want to be cheap. After all, Ansu Fati is still the same as other players in general, apart from being less prominent, he is also unable to maintain his position, where a 20 year old player is mature enough as part of the main squad. Moreover, currently Barcelona has no shortage of talented players for next season, they have invested in new young players to be brought in.