Look at Christian Vasquez's eye -- which he's LUCKY to still have after getting hit directly in the face with a baseball during a practice sesh.

The Boston Red Sox catcher revealed his scar Monday during a media session and explained how wearing sunglasses may have saved his vision ... and the whole thing is super scary.

Vasquez was working on drills on one of the practice fields at Boston's JetBlue Park complex Thursday when pitcher Hirokazu Sawamura fired a ball his way ... mistakenly thinking Vasquez was looking in his direction.

We know what you're thinking ... and no, video of the incident has not been released.

The ball struck 30-year-old Vasquez with such force, it opened up a nasty gash under his left eye -- requiring immediate medical attention.

Vasquez required several stitches to close the wound -- and says if it wasn't for the sunglasses, he might have lost his left eyeball.

"You like my scar," Vasquez joked with reporters Monday before giving a health update.

"I’m doing good ... “My eye is good. I can see perfect."

Vasquez then walked reporters through the incident -- "I saw the ball right here in my face and it knocked me down to the floor."

"The sunglasses saved my eye,” he said ... "If I don’t have the sunglasses, it was a different story."

And, get this ... Vazquez says he's "99% sure" he'll be ready to play Thursday for Boston's Opening Day game against the Baltimore Orioles.