en.wikipedia.org/wiki/William_Fichtner

The released 2014 film credits do not list William Fichtner as Shredder. The American Film Institute's catalog for Teenage Mutant Ninja Turtles lists William Fichtner — Eric Sachs and separately lists Tohoru Masamune — Shredder. Rotten Tomatoes' cast listing for the same film likewise identifies Fichtner as Erich Sachs.

So the role entry "Eric Sacks / The Shredder" is inaccurate for the movie itself. At most, it reflects early-development or tie-in confusion, but in the finished theatrical film Fichtner's credited role is Eric Sachs/Sacks, while Shredder is played by Tohoru Masamune.