You do not have to connect to an external monitor to check the grahics card.
Just take a screenshot (cmd-shift-3).
If it is OK, there is no problem with the graphics processor,
but with one of the LVDS connectors, or with the display itself
Parsec is right. The problem can occur by cutting too deep when removing the display.
The most critical area appears to be the top of the display, because this is where the controller is connected to the LCD by flex cables that are exposed, and you could cut off the fragile joints from the cables to the LCD.