The inner tube on my bikes rear wheel is flat. I pumped it up about 4 days before and rode it a couple days than took a break and when I went to ride it i noticed the air was all gone. I found a small slash on the inside right where this thin piece of plastic keeps the ends of the spokes from poking it, was moved aside so i suspect rubbing against the spoke did it. Should i move that platic thing back over the spokes or buy a better one?

what's the official name for it as well if i need to buy anouther?

Edit: I've also noticed for about a week before the back loses pressure a small bit quicker than the front