Yes you can just replace the LCD panel on Retina models (a lot of people say it is not possible, however it is, just fairly difficult). We do them all the time in the UK, the glass and LCD are stuck together on this model.
Your problem would be with the backlight sheets however; liquid has hit the bottom layer of the backlight sheet and stained it, which usually causes diagonal bright streaks or specks. The LCD panel itself hasn't got liquid in between it (it is completely sealed), may have a bit of liquid on the back of it, but the issue will definitely be the sheets. The sheets can't really be cleaned unfortunately, so the whole lid needs to be replaced. You can buy new backlight sheets, but you have to removed the old screen without cracking it (pretty difficult) and the new sheets are usually dirty, since there is no easy way to handle them.