Expecting Luck to be the next Peyton Manning is a reasonable expectation?
If expecting a guy to be the one of the best ever is reasonable, I'm not sure I know what an unreasonable expectation is.
As reasonable as it is to expect Newton to be the next Michael Vick (or Jamarcus Russell, depending on your perspective).
Any expectations for Manning to become Marino were unreasonable as well. Manning has obviously surpassed those expectations, but that doesn't make them reasonable. Expecting a college player (Luck or Tennessee era Manning) to become one of the best QBs of all time is not reasonable. Luck will probably be a very good NFL Qb, but how is the expectation of him becoming a first ballot hall of famer any where close to being reasonable?
Expecting extraordinary, everything has to go right, odds stacked against it kind of results is not reasonable.
It's talent evaluation, and it happens on an annual basis. Some people are good at it. Some aren't. That's why some teams draft well and others don't.
If a kid in college has the same skill set, intelligence and work ethic as Player X had when he was in college, why would it be unreasonable for scouts to say he's the next Player X?