There’s zero evidence to suggest he won’t become “effective”, in fact with 2 years of coaching from Heupel and Halzle there is more evidence that he will in fact continue to improve. There is undeniable potential for him to be a very good QB, Heupel & Halzle have proven they have the ability to develop QBs. Players are not stuck where they are; they practice, they develop, they improve from day to day, week to week, month to month, year to year; the only limiting factor is the desire to improve.
Also, it is interesting you brought up Peyton Manning… his freshman year, came into the first game as QB 3 (technically 3a w/ Brandon Stewart being 3b), he was behind Jerry Colquitt and Todd Helton. The plan was not for Peyton to play in that game but unfortunate injuries thrust him in to action early; in fact if I recall correctly I seem to recall an interview in which either Archie or Olivia stated they were told Peyton was to red shirt and spend time in a college weight training program before seeing the field. That is not an uncommon occurrence, even for top QB recruits. There is a big difference between high school and college.
I don’t think most recruits think they have to come in and be a superstar on day 1. Especially ones who are interested in the team, rather than self. Most want to come in and learn and prepare for when their number is called.