Why can't the Jaguars win with Blaine Gabbert?

Now that it's not just me saying that Jacksonville Jaguars rookie quarterback Blaine Gabbert has a legitimate chance to overtake veteran quarterback David Garrard for the starting position, there's a false pretense that's consistently floating around that just doesn't make sense. There seems to be this belief that if David Garrard is the starter in 2011 the team will compete for the playoffs, but if Blaine Gabbert starts they're a 5-6 win team.

If Blaine Gabbert wins the opening day job however, that means he's as good or better than David Garrard as a rookie.

So, logically speaking... If Gabbert is as good or better, why can't the Jaguars win with him? Theoretically they would be playing a better player, right?

Is being a rookie quarterback really the difference between 9-7/10-6 and 5-11/6-10? I can't believe that a player just being a rookie is worth a 3-5 game swing in the win column. Will he make rookie mistakes that might cost the Jaguars a game? Absolutely. But will it cost the Jaguars 3-5 wins through the course of the season? Highly doubtful. We have to remember again, if Gabbert wins the starting job it means that you're starting the better player. I just don't see how starting the better player makes the team that much worse, just for the sole fact that they're a rookie.

Personally, I just don't see this team as a playoff caliber team regardless of who the quarterback is. The competition for the starting quarterback position is very real, however. It still has to play out over the course of the preseason, however.