Throttle cable rubber cover on handle bars

[Arfa]

On the throttle cables just before they go into the throttle grip on the bars, there's a little rubber cover for the join between the metal housing and the two cables the come off from it. I've gone and shredded this on my last off. Picked up a whole throttle cable assembly off ebay cheap, but I can't see how I can swap over this rubber cover on it's own, there's just too much plastic cable guides and stuff in the way to slide it off. Am I being daft, or do I really need to swap the entire length of the throttle cables to change this rubber bit?