Mankey can sweep Brock without items, but it needs to be at a reasonable level to ensure a sweep. A level 9 Mankey can lose if you're unlucky, although it will still leave a mark for something else to finish him off. Having a Potion on you is a good idea, and there are at least 4 of them available for free before you reach Brock so you should have some. But yeah I agree that the fact that Yellow Mankey is an excellent partner for Pikachu early on should be mentioned. I've played Yellow a lot and I often use Mankey (or Butterfree, but I've taken a liking to Mankey lately) at least initially to get around Brock, even if I don't necessarily keep using it for the whole game.