Haha, the classic 'Faceplant' issue! Don't worry, this plagues everyone (even artists) because Unity uses aY-Upcoordinate system while Blender usesZ-Up.
When you copy the animation data from the imported FBX to your existing Armature, Blender tries to compensate for that 90-degree axis difference, but often applies it twice or not at all depending on your settings.
Here is the Engineering Fix:
Method 1: The Import Fix (Prevention)
When importing the FBX into Blender, look at the Transform settings on the right side of the file browser:
Set Manual Orientationto:Forward = -Z ForwardandUp = Y Up.
Important:Check the box"Force Connect Children"->OFF.
Check"Apply Scalings"->FBX All.
Method 2: The 'Delta' Fix (If already imported)
If you apply the animation and your character snaps face-down:
Select your Armature object in Object Mode.
Go to theObject Propertiestab (Orange Square icon).
Look atTransform > Rotation. If it says X: -90 (or 90), change it to 0.
If that doesn't work, look slightly lower at theDelta Transformdropdown. Sometimes Blender hides the import rotation offset there. Zero that out too.