He was probably familiar with Alexander Pope's version, which was the prestigious version at the time.1 It's likely that Keats wasn't enamored with this version. He was famously critical of 18th century poets, thinking their poetry too rule-bound and stifled. He expresses some of these criticisms in the poem Sleep and Poetry:1
Could all this be forgotten? Yes, a sc[h]ism
Nurtured by foppery and barbarism,
Made great Apollo blush for this his land.
Men were thought wise who could not understand
His glories: with a puling infant’s force
They sway’d about upon a rocking horse,
And thought it Pegasus. (Sleep and Poetry, 181-187)
In this passage, Keats compares 18th century poets to infants riding upon a wooden horse, thinking it to be Pegasus. That's a pretty biting metaphor. This criticism made Byron quite angry with him.
But ye were dead
To things ye knew not of,—were closely wed
To musty laws lined out with wretched rule
And compass vile: so that ye taught a school
Of dolts to smooth, inlay, and clip, and fit,
Till, like the certain wands of Jacob’s wit,
Their verses tallied. Easy was the task:
A thousand handicraftsmen wore the mask
Of Poesy. Ill-fated, impious race!
That blasphemed the bright Lyrist to his face,
And did not know it,—no, they went about,
Holding a poor, decrepid standard out
Mark’d with most flimsy mottos, and in large
The name of one Boileau! (Sleep and Poetry, 193-206)
In the above, the same criticism is repeated in the lines "were closely wed / To musty laws lined out with wretched rule / And compass vile". Keats attributes the deterioration of English poetry partly to Boileau, a French poet and critic of late 17th century. Boileau published a famous book, L'Art poétique, which influenced English literature.
It might be useful to compare Pope's version with that of Chapman's.1 The below passages correspond to the same account in the Iliad, a description of Diomedes' armor and ferocity. You can see how Chapman's line had more syllables (fourteen). His line is also a lot freer, not always punctuated at the end as we find in Pope.
High on his helm celestial lightnings play,
His beamy shield emits a living ray;
Th’ unwearied blaze incessant streams supplies,
Like the red star that fires th’ autumnal skies,
When fresh he rears his radiant orb to sight,
And bathed in Ocean shoots a keener light.
Such glories Pallas on the Chief bestow’d,
Such, from his arms, the fierce effulgence flow’d:
Onward she drives him, furious to engage,
Where the fight burns, and where the thickest rage. (Pope's Iliad)
From his bright helm and shield did burn a most unwearied fire,
Like rich Autumnus’ golden lamp, whose brightness men admire
Past all the other host of stars, when, with his cheerful face
Fresh wash’d in lofty Ocean waves, he doth the skies enchase.
To let whose glory lose no sight, still Pallas made him turn
Where tumult most express’d his pow’r, and where the fight did burn. (Chapman's Homer)
In summary, I think that Keats was well aware of Pope's translation before reading Chapman. But I'm inclined to think that Keats was not enamored with Pope's translation, as, by his own admission, it wasn't until reading Chapman that he gained an appreciation for Homer.
1 Longman Anthology of British Literature, Volume 2A