Changing Base Sprite on a GameObject which has Sprite Switching animations

Hey there,

Here what I want to do ( as shown on the picture ) :

I have a game object ( MONSTER ) , Composed with multiple sprites, ( BODY & MOUTH) which has an IDLE animation and an ATTACK animation.
the IDLE animation only contain position/rotation/scale anim.
the ATTACK animation has in addition a sprite switching animation.

it works well up to this point.

Now I want to change the base Sprites of the MONSTER at runtime, depending on how it is wounded.
the sprites BODY_02 & MOUTH_02 are the wounded versions.

Now I have a problem :

I've tried Layers, BlendTree, SubState, changing sprite in code at runtime...
The BODY sprite - which is not updated in any animation - is changing well, but the MOUTH sprite is always rewrote by - I think - Mecanim, even if I never start the ATTACK animation.

Has anyone tried something like that before?
I have some kind of workaround which consists of separating animated sprites and "static" sprites then hide/reveal them in animation instead of switching the sprite.. but I can't do transitions like that.