Yeah, the best bet is to get a replacement.
But if you want to save the hassle of the drive to hardware store, and if the skimmer is not a heck to put together and take apart, you may want to take a look at the ring and inspect it for crack (pull it apart like a rubber band to see the minute cracks), and if it look brand new, try to clean it and lub it, and put it back together carefully (o-ring may leak if over tightened), and inspect for leak.
But for a few dollars, you probably want to assume it is bad unless it is a difficult size to match. Ace Hardware usually have a good selection of these parts. Good luck.