Howdy, welcome to TPT! Has the tetra lost both gill covers, or just the one? The loss of gill covers isn't an early symptom of any common tetra disease, so it's more likely that they were torn off by another fish; although it would seem unusual that both covers are missing. Is the fish displaying any other abnormal traits or behavior?
One neat thing about tetras is that they will refuse to school with any among them that are sick, effectively reducing the spread of disease. This can also act as an early indicator of sickness, even if other symptoms go unnoticed. If not yet quaratined, is this tetra schooling with the others you brought home?