Bayer were confirmed champions on Sunday, so interrupting Bayern Munich's dominance after 11 years.
Former Real Sociedad B coach Xabi said amongst the celebrations: "This title also belongs to my predecessors: Christoph Daum, Klaus Toppmller, Roger Schmidt and many others.
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"I am very proud to be part of the history of Leverkusen."
He also said: "This is a very special moment for the club.
"Winning the Bundesliga for the first time after 120 years of history is something very special. The players are of the highest level, they are an incredible team. I am very proud of everyone.
"It is an honour for me to work here."