In Chamber of Secrets, Oliver Wood mentions that the Gryffindor team lost the game and the Quidditch cup all because Harry, the team Seeker, was in the hospital wing, recovering from saving the day. It seems to imply that without Harry, there was no Seeker, which would be ridiculous. I don't know much about sports, but I think they usually have players ready to step in as a replacement should someone become injured or something.
Does Gryffindor have a backup Seeker, and if not, why not?