Firmino has been in fine form this season, scoring 13 goals in 25 Bundesliga appearances.
Having helped Hoffenheim to 10th in the Bundesliga this term, alongside the likes of Kevin Volland and Anthony Modeste, Firmino has caught Keller's eye, with the Schalke boss hinting at an interest.
"Firmino is a good player and could help us," Keller told Sport Bild.
"He is a player of great quality and would certainly be interesting."
Firmino, 22, moved to Hoffenheim in 2011 from Brazilian side Figueirense and has been linked with a host of Europe's top clubs such as Liverpool and Chelsea.
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