Manager Markus Gisdol has over-hauled his squad after narrowly missing out on relegation last season.
He has told a number of high-profile players, including international goalkeeper Tim Wiese, that they are surplus to requirements.
"This is a question of law and morality," Metzelder told Sky.
"When two sides sign a contract, you have to stick up for it."
Hoffenheim drew 2-2 with Nuremburg yesterday, on the opening day of the Bundesliga.