When I got back from a ride the other day I noticed that a small O-ring type seal had fallen out of the lower idler pulley of my old R600 tri-color rear derailleur. It looks like it seals the bearing in this pully to keep road grime out. It is ripped so it can't be reused. Do I need to replace it? Can I find a replacement part? Sorry if I'm not describing the issue well, I'm new to the sport and this happened after my second ride.