I do recall some talk about the wrist injury even in 2009...especially when he wasn't "Big Papi" when he returned in 2008. But to be fair, often the decline of a player can be triggered by an injury and then an inability to stay healthy or just keep declining to a point where they never return to their pre-injury form. But, you are right that it often gets overlooked in the whole context of his career, perceived fork in his back, and then subsequent resurgence.
One of Papi's most desirable traits has been his ability to stay healthy late in his career. Really only the achilles injury in 2012 in the past 6 seasons. Obviously being a DH helps, but even taking that into account, it's impressive.
