The most common eye injury is the result of the guinea pig rooting around in the hay and getting poked in the eye by a piece of it. If you notice a guinea pig with a watering eye check the surface of the eye for any scoring. Obviously, also check at the same time to see if there is any foreign body in it. More often than not it is not until the eye goes opaque about a day after the injury that it is noticed and by this time the eye is well on the mend. A dab of Brolene eye ointment is recommend and the eye should be monitored for a few days of course.
Sometimes the eye can become very badly infected and bulge out of it's socket if it is not noticed and treated early. A huge ugly scab can form on it and it looks very inflamed. Even this can very effectively treated by antibiotic drops and injections. So long as the infection stays encapsulated within the eye ball and does not spread behind then the animal's life is not at hazard.
If the infection does get behind the eye then serious thought must be given to having the eye removed surgically, and very quickly indeed. The removal of an eye is the least risky of all operations. Everything depends of course on how quickly it is done or how far the infection has spread. Needless to say infections of this nature have to be dealt with by a vet.