Scratch and Scrapes on AP

Will the severe scratch and scrapes on the front antenna side of 5750AP cause problem with signal?

in the plastic? no

Isn’t that plastic cover also serve as an antenna? Or is the antenna inside the casing?

Antenna is mounted to the board inside. The plastic is just the housing.

It might be of interest to pull the board from the housing, it’s very easy. Be sure to observe proper ESD grounding procedures.