MK2 Door Glass

Going to replace front passenger  door on the car mk2 Fabia, have purchased a replacement door and have changed all bits onto new door except the glass because there was a plastic runner that was stuck to the glass at the right hand top corner of the glass  but has broken off, it seems to be a sort of guide that's stuck to the glass and then goes into runner on the door frame, contacted dealer and they said glass has to be replaced as this runner comes with the glass, would the glass run up and down without this bit of plastic, when glass is in place you cannot see this runner bit but not sure how important this small plastic clip thing is