A noteworthy power the rings "do not" have is the ability to automatically heal injuries, though they can provide shielding. In Hal Jordan's origin story, Abin Sur passed on his ring to Hal because he was unable to treat his own fatal injuries. If the Green Lantern happens to be a skilled physician, then the ring can be invaluable as it can conjure any conceivable medical tool, but it cannot do much for a Lantern who lacks medical expertise. When Hal Jordan breaks his arm, the best he can do is conjure up a cast. This is further extended into an ability to replace large sections of one's injured body with constructs, but this too requires detailed biological knowledge of one's body and concentration enough to prolong the construct.