This is not an argument against Brock, he has been great considering everything he and the 49ers have been through this year.It took Montana 11 years before having a season average passer rating higher than Brock is averaging this year. And he only did it twice in his career and one of those seasons he played 1 game and threw 126 yards. Brock had the highest passer rating in the NFL right now.
But the game of football is completely different now, I can't imagine what Jerry Rice could do in today's NFL where there he can't be touched after 5 yards. He would be impossible to cover, he was impossible to cover then and you could grab, move him. Montana's passer rating is only relevent to compare him to his peers in his era. The game if football is totally different and todays QBs get a much better world to work in.
That said, Brock has been great, has room to get better as he is still adjusting to the game. If he can make a run/look good in the playoffs he should be a starter next season. In the NFL you physical tools can only get you so far, the head on the shoulders that makes the difference.