The Brazilian forward joined the Russian side last summer despite reported interest from Chelsea, and the former Porto man reveals that the package put together by Zenit was too good to refuse.
"I will not lie, people know I am a footballer and I had to seize the moment with Zenit's financial offer," he told Euronews.
"I wanted a new goal to achieve and it had to be in a different league having already won everything in Portugal [with Porto].
"I received various proposals from teams in England and Spain but we never reached an agreement."
Zenit host Liverpool in the Europa League on Thursday.