As Barca lost 4-2 at Girona on Saturday, Real's victory at Cadiz saw them confirmed as LaLiga title winners.
Xavi said: "They are rightly champions. But now we have to think about giving our best in these last four matches."
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On defeat at Girona, he added: "I think it's a mental thing. We prepare the matches very well, but then we make sensational mistakes that have marked our season. The hour we did was brilliant, but the result doesn't make you right at all.
"The last 20 minutes were not up to the level of the Barcelona players."