This is more "if they're not perfect from jump fans dont want them".
How many NFL teams even have fullbacks? How many college teams at the level they'd be drafted these days? It's not many.
Just like every other position there's this expectation of excellence that is unachievable. Especially instantaneously. The kid wasn't even a fullback in college -- he's learning the position.
If this is a jet pilot or a surgeon -- I get it, the trainee = bad idea. But this is entertainment...professional, but still just a place where on the job training is the REALITY of the business. He can't improve where there's no padded, full speed practice anymore. He can't improve in college as a primer because the college game builds maybe 1-2 a YEAR that are professional level.
He pancaked the Jets player. Cleared him the *bleep* out. For once why don't we say "nice work kid. Whatever you did there, keep doing that because it shows you're getting better"?