The article for SEO is 'an' ie. 'an SEO' but Search Engine Optimizer ( Abbreviation of SEO ) is called as 'a Search Engine Optimizer'. Why do these variations?
Because when you pronounce 'SEO', it starts with the vowel 'es' (listen to it), in Hindi- 'ए' (एसीओ). The rule of articles apply the way we 'pronounce' the word.
When you write it the full term - Search engine optimizer, you pronounce each word differently so it is 'a search engine optimizer' (अ सर्च एंजिन ओप्टिमाईज़र)
Even further, if you pronounce SEO and have a noun following it, you still use 'an' - I know Tim who is 'an SEO expert'.
So, to conclude...
You certainly require an SEO
A search engine optimizer helps us bring our site up on SERPs
Tim is an SEO expert