I think it's certainly believable. If you remove the scene where Sachs talks to Shredder (calling him master) and sub-out that lab sequence with Sachs, Vernon, and April that's all it would take IIRC. I think that's where he mentions Shredder raised him, too.
If you remove the lab scene you also take out the need to remove the mutagen from the blood to made the antidote for the poison. Then you have to alter the scene of them saving Splinter at the endd. It's hard to remove one scene unless you remove a good half dozen scenes as well and many of them have visual affects in them that have to be budgeted and storyboarded ahead of time. There's no hint of any extended refilming or post production time.
I didn't say remove - I said sub-out. Someone mentioned in the B-roll footage there's another scientist in place of Sachs.
(I didn't watch all of the footage so I don't know)
And yet he admitted be cast as Sachs, there's nothing other than his conflicting statements to suggest that he was going to play Shredder.