William Fichtner confirms Eric Sacks was Shredder originally until reshoots

[Candy Kappa]

Originally Posted by Coola Yagami

That just means he doesn't remember life as a normal rat, you know, like how the turtles don't remember their thrilling adventures crawling around in a glass bowl in a pet shop.

Probably, but he is an adult rat, so he could have had some time before the lab experiments, the turtles looks very juvenile.