The 21-year-old enjoyed an impressive debut season with the Bundesliga club in 2018/19, scoring 14 goals in 34 league appearances.
L’Equipe reports that Saints and Newcastle are among the clubs to have submitted bids in the region of €20-25 million for the player, but every approach has been turned down.
Mainz signed Mateta for €10 million last summer from Lyon and are reluctant to let him go so soon.
The outlet believes that an offer of €35-40 million could be required to secure the services of the France Under-21 international.
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