What happened is a combination of being limited in ability, god awful Coaching at QB/OC & extremely predictable play calling. Rosier was a low rated Al Folden recruit coming out of HS, that had mechanical issues in his throwing motion that didn't actually watch film or study until 2017 by his own admission. He actually overachieved a lot more than anyone would have ever expected.
But a lot of people always remove all context when discussing the 2017 season & just look at the numbers on its face, but that's part of the problem as to why people were so shocked he fell off a cliff in 18. If you closely examine the 17 season & rewatch the games, you'll see he was a lot closer to be as bad as he was in 18 in the 17 season than you might think, despite the overal Win/Loss record.
In 2017, his best games were Toldeo, GTech & Cuse, the rest of the games he was serviceable enough for us to win combined with having great support in big playmakers around him & an unappreciated Defense.
He only had 3 games with more than 20 completions, was a 54% passer on the season & threw INT's in 8 out of 13 games, people just overlooked that fact because we won multiple games regardless of how bad he looked sometimes because he wasn't asked to carry the team.
Against FSU, all anyone remembers is the throw to Langham, but he finished that game 19/44 254yds 3TD's 1INT at 43.2%cmp, which objectively is below average but because of the way in which we won vs a hated rival people don't care. That's kinda the point, as long as we won his play didn't really matter to most lol.
Vs UNC was another perfect example of this, he completed 42% of his passes & was terrible, but had a bunch of big plays from Berrios, JT4, Homer, Deejay & Herndon, so everyone just blamed the poor performance of the game on the Defense. Despite the fact the Defense forced 3INT's & a Fumble while also literally knocking out 3 QB's in the game but the Offense was just so bad for most of the game UNC was able to hang around until late.
Even when we dominated Va Tech, he was 11/22 202yds 2TD's passing 1TD rushing with 3INT's at 50%cmp. Again, looking at it objectively those are not good numbers for a P5 QB, but our Defense was just so on fire for that game, his reckless turnovers didn't affect us negatively because we were just completely dialed in & winning that game regardless.
His best game where he looked like a **** good QB was vs Cuse, that's one of the only times where his play exceeded the level of talent around him, that game he was one of the best players on the field.
But what happened to Rosier is Defenses caught up to how simplistic our offensive game plan was & saw on film all Rosier's flaws & forced him to make throws he simply couldn't make. Once Defenses realized how easy it was to figure out our sequences & that the QB was only going to certain parts of the field, he experienced something called regression to the mean.
Richt did an awful job in preparing this team for the 18 season & Rosier got caught being forced to play above his ability, something he couldn't duplicate again.
