The 20-year-old, who has scored 19 goals in 66 appearances for Botev since making his debut in 2011, is considered one of the best young players in the Balkan country with several European clubs linked with him in the last few months.
Mainz are fourth in the Bundesliga standings with maximum points from three games.
(Reporting by Angel Krasimirov, Editing by Tom Bartlett)
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