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As fans we got a glimpse of the Patriot system working without Tom Brady in 2008. After the 2007 MVP suffered a season-ending knee injury, the starting job fell to unheralded Matt Cassel.
Cassel's entrance into the NFL mirrored Brady's in many ways. After a career backing up Carson Palmer and Matt Leinart at USC, Cassel was picked up by the Pats in the seventh round of the 2005 draft.
While slightly more gifted physically than Brady (he sometimes played tight end, wide receiver or even running back for SC), Cassel made it on to a few teams' radars after a successful combine but wasn't considered starting material. He was placed third on the Patriots' depth chart in 2005.
Backing up arguably the NFL's finest quarterback naturally gave Cassel few chances to showcase his abilities, but when the call came in 2008 he was ready. After seven years as a backup, Cassel started in the Patriot's remaining 15 games, putting up eerily Brady-like numbers in the process.
In the first four years as the Patriots starter (2001-2004 when the Pats won three Super Bowls), Brady averaged 3,480 yards passing, a 61.8 completion rate, an 87.7 passer rating and 24 touchdowns.
In his what was to be his only season as starter in New England, Cassel threw for 3,693 yards, a 63.4 completion rate, an 89.4 passer rating and 21 touchdowns.
Since then, Cassel's success as the starter for Kansas City has been sporadic. While he echoed the success with New England in leading the Chiefs to a division title, his 2009 season finished a lackluster 4-12. 2011 started slowly, but after two consecutive wins Kansas City is 2-3. Cassel is currently 27th among quarterbacks with only 945 passing yards.
In the New England system he was great, but outside of it Cassel has been mortal. Maybe this is proof enough for some that Brady is merely the product of Belichick's system, but I say we go further.
It's clear that quarterbacks who might have been average outside the system excelled in it, but would it be the same for Tom Brady? Would he have had the same career outside of New England?