Madrid claimed a third consecutive Champions League title on Saturday, beating Liverpool 3-1 in the final in Kiev .
Barcelona, meanwhile, secured the La Liga crown for the third time in four seasons during 2017-18.
But Xavi – who won four Champions League trophies with Barca – accused the Spanish champions of becoming "dormant", despite signing the likes of Luis Suarez, Philippe Coutinho and Ousmane Dembele in recent seasons.
"Madrid have a winning gene and Barca have fallen asleep," he told El Penalti .
"They have been dormant for years with signings and that takes its toll."
Xavi also pointed to Madrid's luck in the Champions League , the latest of which was Mohamed Salah's injury and Loris Karius' errors in the final.
"Real Madrid had everything in sight. Injured players like [Paris Saint-Germain star] Neymar, [Bayern Munich's Arjen] Robben, [Jerome] Boateng, the referees," he said.
"The most important Liverpool player was injured, Bayern goalkeepers [Manuel Neuer injured and Sven Ulreich's semi-final error] and Liverpool.
"It seems like a witch, but the issue is that Barcelona have fallen asleep and it seems Barca are winning titles, but the others are winning the Champions League."