The midfielder, who signed for Chelsea for £27million on Thursday, scored 42 goals in 151 matches for the Spanish giants.
But the club claim there was a "downward trend in his stats every season" and that Fabregas "was never as good in the second half of a season as in the first".
A statement on the club's official website said: "Despite glowing starts to each campaign, Cesc’s contributions to the cause gradually decreased as each season drew to a close.
"From being someone who joined in with the attack, supplying and scoring goals, the magic tended to fade later on in each season. He only scored one, six and one goals in the last 24 games of each season.
"For some reason, he was never as good in the second half of a season as in the first."
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